Add a simple selftest for a move_mount(MOVE_MOUNT_SET_GROUP). This tests
that one can copy sharing from one mount from nested mntns with nested
userns owner to another mount from other nested mntns with other nested
userns owner while in their parent userns.

  TAP version 13
  1..1
  # Starting 1 tests from 2 test cases.
  #  RUN           move_mount_set_group.complex_sharing_copying ...
  #            OK  move_mount_set_group.complex_sharing_copying
  ok 1 move_mount_set_group.complex_sharing_copying
  # PASSED: 1 / 1 tests passed.
  # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

Link: https://lore.kernel.org/r/20210715100714.120228-2-ptikhomirov@virtuozzo.com
Cc: Shuah Khan <shuah@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Mattias Nissler <mnissler@chromium.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: lkml <linux-kernel@vger.kernel.org>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>