ceph: clean up locking annotation for ceph_get_snap_realm and __lookup_snap_realm

ceph: clean up locking annotation for ceph_get_snap_realm and __lookup_snap_realm

BugLink: https://bugs.launchpad.net/bugs/1944202
commit df2c0cb7f8e8c83e495260ad86df8c5da947f2a7 upstream.

They both say that the snap_rwsem must be held for write, but I don't
see any real reason for it, and it's not currently always called that
way.

The lookup is just walking the rbtree, so holding it for read should be
fine there. The "get" is bumping the refcount and (possibly) removing
it from the empty list. I see no need to hold the snap_rwsem for write
for that.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>