ceph: decoding error in ceph_update_snap_realm should return -EIO

Currently ceph_update_snap_realm returns -EINVAL when it hits a decoding
error, which is the wrong error code. -EINVAL implies that the user gave
us a bogus argument to a syscall or something similar. -EIO is more
descriptive when we hit a decoding error.

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