fs/9p: fix readdir()

Al Viro's IOV changes broke 9p readdir() because the new code
didn't abort the read when it returned nothing. The original
code checked if the combined error/length was <= 0 but in the
new code that accidentally got changed to just an error check.

Add back the return from the function when nothing is read.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Fixes: e1200fe68f ("9p: switch p9_client_read() to passing struct iov_iter *")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/9p/vfs_dir.c

@@ -138,6 +138,8 @@ static int v9fs_dir_readdir(struct file *file, struct dir_context *ctx)
 					   &err);
 			if (err)
 				return err;
+			if (n == 0)
+				return 0;
 
 			rdir->head = 0;
 			rdir->tail = n;