ceph: fail the request if the peer MDS doesn't support getvxattr op

Just fail the request instead sending the request out, or the peer
MDS will crash.

Link: https://tracker.ceph.com/issues/56529


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>