nfsd: use vfs_fsync_range(), not O_SYNC, for stable writes

NFSv4 shares the same struct file across multiple writes.  (And we'd
like NFSv2 and NFSv3 to do that as well some day.)

So setting O_SYNC on the struct file as a way to request a synchronous
write doesn't work.

Instead, do a vfs_fsync_range() in that case.
Reported-by: Peter Staubach <pstaubach@exagrid.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.c

@@ -1024,11 +1024,6 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file,
 
 	if (!EX_ISSYNC(exp))
 		stable = 0;
-	if (stable && !use_wgather) {
-		spin_lock(&file->f_lock);
-		file->f_flags |= O_SYNC;
-		spin_unlock(&file->f_lock);
-	}
 
 	/* Write the data. */
 	oldfs = get_fs(); set_fs(KERNEL_DS);
@@ -1044,8 +1039,12 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file,
 	if (inode->i_mode & (S_ISUID | S_ISGID))
 		kill_suid(dentry);
 
-	if (stable && use_wgather)
-		host_err = wait_for_concurrent_writes(file);
+	if (stable) {
+		if (use_wgather)
+			host_err = wait_for_concurrent_writes(file);
+		else
+			host_err = vfs_fsync_range(file, offset, offset+*cnt, 0);
+	}
 
 out_nfserr:
 	dprintk("nfsd: write complete host_err=%d\n", host_err);