Don't feed anything but regular iovec's to blk_rq_map_user_iov

In theory we could map other things, but there's a reason that function
is called "user_iov".  Using anything else (like splice can do) just
confuses it.
Reported-and-tested-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

block/blk-map.c

@@ -118,6 +118,9 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 	struct iov_iter i;
 	int ret;
 
+	if (!iter_is_iovec(iter))
+		goto fail;
+
 	if (map_data)
 		copy = true;
 	else if (iov_iter_alignment(iter) & align)
@@ -140,6 +143,7 @@ int blk_rq_map_user_iov(struct request_queue *q, struct request *rq,
 
 unmap_rq:
 	__blk_rq_unmap_user(bio);
+fail:
 	rq->bio = NULL;
 	return -EINVAL;
 }