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: J. R. Okajima <hooanon05g@gmail.com>
Fixes: 18290650b1c8 ("NFS: Move buffered I/O locking into nfs_file_write()")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 7d62097..ca699dd 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -657,7 +657,10 @@
 	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 */