ceph: allow rename operation under different quota realms

Returning -EXDEV when trying to 'mv' files/directories from different
quota realms results in copy+unlink operations instead of the faster
CEPH_MDS_OP_RENAME.  This will occur even when there aren't any quotas
set in the destination directory, or if there's enough space left for
the new file(s).

This patch adds a new helper function to be called on rename operations
which will allow these operations if they can be executed.  This patch
mimics userland fuse client commit b8954e5734b3 ("client:
optimize rename operation under different quota root").

Since ceph_quota_is_same_realm() is now called only from this new
helper, make it static.

URL: https://tracker.ceph.com/issues/44791


Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>