NFS: Fix error reporting in nfs_file_write()

When doing O_DSYNC writes, the actual write errors are reported through
generic_write_sync(), so we must test the result.
Reported-by: NJ. R. Okajima <hooanon05g@gmail.com>
Fixes: 18290650 ("NFS: Move buffered I/O locking into nfs_file_write()")
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/file.c

@@ -657,7 +657,10 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 	if (result <= 0)
 		goto out;
 
-	written = generic_write_sync(iocb, result);
+	result = generic_write_sync(iocb, result);
+	if (result < 0)
+		goto out;
+	written = result;
 	iocb->ki_pos += written;
 
 	/* Return error values */