caif: don't assume iov_iter type

The details of the iov_iter types are appropriately abstracted, so
there's no need to check for specific type fields. Just let the
abstractions handle it.

This is preparing for io_uring/net's io_send to utilize the more
efficient ITER_UBUF.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20230111184245.3784393-1-kbusch@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: keithbusch

net/caif/caif_socket.c

@@ -533,10 +533,6 @@ static int caif_seqpkt_sendmsg(struct socket *sock, struct msghdr *msg,
 	if (msg->msg_namelen)
 		goto err;
 
-	ret = -EINVAL;
-	if (unlikely(msg->msg_iter.nr_segs == 0) ||
-	    unlikely(msg->msg_iter.iov->iov_base == NULL))
-		goto err;
 	noblock = msg->msg_flags & MSG_DONTWAIT;
 
 	timeo = sock_sndtimeo(sk, noblock);