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>