start adding the tag to iov_iter

For now, just use the same thing we pass to ->direct_IO() - it's all
iovec-based at the moment.  Pass it explicitly to iov_iter_init() and
account for kvec vs. iovec in there, by the same kludge NFS ->direct_IO()
uses.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/btrfs/file.c

@@ -1740,7 +1740,7 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb,
 		goto out;
 	}
 
-	iov_iter_init(&i, iov, nr_segs, count, 0);
+	iov_iter_init(&i, WRITE, iov, nr_segs, count);
 
 	err = file_remove_suid(file);
 	if (err) {