fs: implement vfs_iter_write using do_iter_write

De-dupliate some code and allow for passing the flags argument to
vfs_iter_write.  Additionally it now properly updates timestamps.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/splice.c b/fs/splice.c
index 540c4a4..ae41201 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -762,7 +762,7 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
 
 		iov_iter_bvec(&from, ITER_BVEC | WRITE, array, n,
 			      sd.total_len - left);
-		ret = vfs_iter_write(out, &from, &sd.pos);
+		ret = vfs_iter_write(out, &from, &sd.pos, 0);
 		if (ret <= 0)
 			break;