ceph: allow object copies across different filesystems in the same cluster

OSDs are able to perform object copies across different pools.  Thus,
there's no need to prevent copy_file_range from doing remote copies if the
source and destination superblocks are different.  Only return -EXDEV if
they have different fsid (the cluster ID).

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