ceph: fix memory leak on decode error in ceph_handle_caps

If we hit a decoding error late in the frame, then we might exit the
function without putting the pool_ns string. Ensure that we always put
that reference on the way out of the function.

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