nfsd: warn on odd reply state in nfsd_vfs_read

As far as I can tell this shouldn't currently happen--or if it does,
something is wrong and data is going to be corrupted.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.c

@@ -936,6 +936,7 @@ nfsd_vfs_read(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file,
 			.u.data		= rqstp,
 		};
 
+		WARN_ON_ONCE(rqstp->rq_resused != 1);
 		rqstp->rq_resused = 1;
 		host_err = splice_direct_to_actor(file, &sd, nfsd_direct_splice_actor);
 	} else {