If the userspace process servicing fuse requests is running in
a pid namespace then pids passed via the fuse fd need to be
translated relative to that namespace. Capture the pid namespace
in use when the filesystem is mounted and use this for pid
translation.

Since no use case currently exists for changing namespaces all
translations are done relative to the pid namespace in use when
/dev/fuse is opened. Mounting or /dev/fuse IO from another
namespace will return errors.

Requests from processes whose pid cannot be translated into the
target namespace are not permitted, except for requests
allocated via fuse_get_req_nofail_nopages. For no-fail requests
in.h.pid will be 0 if the pid translation fails.

File locking changes based on previous work done by Eric
Biederman.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>