ceph: return -EIO if read/write against filp that lost file locks

After mds evicts session, file locks get lost sliently. It's not safe to
let programs continue to do read/write.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/super.h

@@ -707,6 +707,7 @@ struct ceph_file_info {
 	struct list_head rw_contexts;
 
 	errseq_t meta_err;
+	atomic_t num_locks;
 };
 
 struct ceph_dir_file_info {