Add a range of selftests for the new mount_setattr() syscall to verify
that it works as expected. This tests that:
- no invalid flags can be specified
- changing properties of a single mount works and leaves other mounts in
  the mount tree unchanged
- changing a mount tre to read-only when one of the mounts has writers
  fails and leaves the whole mount tree unchanged
- changing mount properties from multiple threads works
- changing atime settings works
- changing mount propagation works
- changing the mount options of a mount tree where the individual mounts
  in the tree have different mount options only changes the flags that
  were requested to change
- changing mount options from another mount namespace fails
- changing mount options from another user namespace fails
- idmapped mounts

Note, the main test-suite for idmapped mounts is part of xfstests and is
pretty huge. These tests here just make sure that the syscalls bits work
correctly.

 TAP version 13
 1..20
 # Starting 20 tests from 3 test cases.
 #  RUN           mount_setattr.invalid_attributes ...
 #            OK  mount_setattr.invalid_attributes
 ok 1 mount_setattr.invalid_attributes
 #  RUN           mount_setattr.extensibility ...
 #            OK  mount_setattr.extensibility
 ok 2 mount_setattr.extensibility
 #  RUN           mount_setattr.basic ...
 #            OK  mount_setattr.basic
 ok 3 mount_setattr.basic
 #  RUN           mount_setattr.basic_recursive ...
 #            OK  mount_setattr.basic_recursive
 ok 4 mount_setattr.basic_recursive
 #  RUN           mount_setattr.mount_has_writers ...
 #            OK  mount_setattr.mount_has_writers
 ok 5 mount_setattr.mount_has_writers
 #  RUN           mount_setattr.mixed_mount_options ...
 #            OK  mount_setattr.mixed_mount_options
 ok 6 mount_setattr.mixed_mount_options
 #  RUN           mount_setattr.time_changes ...
 #            OK  mount_setattr.time_changes
 ok 7 mount_setattr.time_changes
 #  RUN           mount_setattr.multi_threaded ...
 #            OK  mount_setattr.multi_threaded
 ok 8 mount_setattr.multi_threaded
 #  RUN           mount_setattr.wrong_user_namespace ...
 #            OK  mount_setattr.wrong_user_namespace
 ok 9 mount_setattr.wrong_user_namespace
 #  RUN           mount_setattr.wrong_mount_namespace ...
 #            OK  mount_setattr.wrong_mount_namespace
 ok 10 mount_setattr.wrong_mount_namespace
 #  RUN           mount_setattr_idmapped.invalid_fd_negative ...
 #            OK  mount_setattr_idmapped.invalid_fd_negative
 ok 11 mount_setattr_idmapped.invalid_fd_negative
 #  RUN           mount_setattr_idmapped.invalid_fd_large ...
 #            OK  mount_setattr_idmapped.invalid_fd_large
 ok 12 mount_setattr_idmapped.invalid_fd_large
 #  RUN           mount_setattr_idmapped.invalid_fd_closed ...
 #            OK  mount_setattr_idmapped.invalid_fd_closed
 ok 13 mount_setattr_idmapped.invalid_fd_closed
 #  RUN           mount_setattr_idmapped.invalid_fd_initial_userns ...
 #            OK  mount_setattr_idmapped.invalid_fd_initial_userns
 ok 14 mount_setattr_idmapped.invalid_fd_initial_userns
 #  RUN           mount_setattr_idmapped.attached_mount_inside_current_mount_namespace ...
 #            OK  mount_setattr_idmapped.attached_mount_inside_current_mount_namespace
 ok 15 mount_setattr_idmapped.attached_mount_inside_current_mount_namespace
 #  RUN           mount_setattr_idmapped.attached_mount_outside_current_mount_namespace ...
 #            OK  mount_setattr_idmapped.attached_mount_outside_current_mount_namespace
 ok 16 mount_setattr_idmapped.attached_mount_outside_current_mount_namespace
 #  RUN           mount_setattr_idmapped.detached_mount_inside_current_mount_namespace ...
 #            OK  mount_setattr_idmapped.detached_mount_inside_current_mount_namespace
 ok 17 mount_setattr_idmapped.detached_mount_inside_current_mount_namespace
 #  RUN           mount_setattr_idmapped.detached_mount_outside_current_mount_namespace ...
 #            OK  mount_setattr_idmapped.detached_mount_outside_current_mount_namespace
 ok 18 mount_setattr_idmapped.detached_mount_outside_current_mount_namespace
 #  RUN           mount_setattr_idmapped.change_idmapping ...
 #            OK  mount_setattr_idmapped.change_idmapping
 ok 19 mount_setattr_idmapped.change_idmapping
 #  RUN           mount_setattr_idmapped.idmap_mount_tree_invalid ...
 #            OK  mount_setattr_idmapped.idmap_mount_tree_invalid
 ok 20 mount_setattr_idmapped.idmap_mount_tree_invalid
 # PASSED: 20 / 20 tests passed.
 # Totals: pass:20 fail:0 xfail:0 xpass:0 skip:0 error:0

Link: https://lore.kernel.org/r/20210121131959.646623-37-christian.brauner@ubuntu.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>