NFS: Sync the correct byte range during synchronous writes

Since commit 18290650 ("NFS: Move buffered I/O locking into
nfs_file_write()") nfs_file_write() has not flushed the correct byte
range during synchronous writes.  generic_write_sync() expects that
iocb->ki_pos points to the right edge of the range rather than the
left edge.

To replicate the problem, open a file with O_DSYNC, have the client
write at increasing offsets, and then print the successful offsets.
Block port 2049 partway through that sequence, and observe that the
client application indicates successful writes in advance of what the
server received.

Fixes: 18290650 ("NFS: Move buffered I/O locking into nfs_file_write()")
Signed-off-by: NJacob Strauss <jsstraus@amazon.com>
Signed-off-by: NTarang Gupta <tarangg@amazon.com>
Tested-by: NTarang Gupta <tarangg@amazon.com>
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/file.c

@@ -631,11 +631,11 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 	if (result <= 0)
 		goto out;
 
-	result = generic_write_sync(iocb, result);
-	if (result < 0)
-		goto out;
 	written = result;
 	iocb->ki_pos += written;
+	result = generic_write_sync(iocb, written);
+	if (result < 0)
+		goto out;
 
 	/* Return error values */
 	if (nfs_need_check_write(file, inode)) {