mds: fix null dereference

Server::handle_client_file_setlock() should return after calling
reply_request()

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Merge pull request #2743 from ceph/wip-mds-coverity

mds: fix null dereference

Reviewed-by: Sage Weil <sage@redhat.com>