nfsd: fix sparse warning in vfs.c

fs/nfsd/vfs.c:991:27: warning: Using plain integer as NULL pointer
Signed-off-by: NHarvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/vfs.c

@@ -988,7 +988,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file,
 	 * flushing the data to disk is handled separately below.
 	 */
 
-	if (file->f_op->fsync == 0) {/* COMMIT3 cannot work */
+	if (!file->f_op->fsync) {/* COMMIT3 cannot work */
 	       stable = 2;
 	       *stablep = 2; /* FILE_SYNC */
 	}