very odd check in caif_seqpkt_sendmsg()

[Al Viro <viro@ZenIV.linux.org.uk>]

This check is very odd:
        if (unlikely(msg->msg_iov->iov_base == NULL))
                goto err;
What happens if we get call it with msg_iovlen being 0?  verify_iovec()
(or rw_copy_check_uvector(), for that matter) is just fine with that -
sendmsg() purely for msg_control is normal on e.g. AF_UNIX sockets.
And we end with ->msg_iov pointing to iovstack[], with iovstack[0] being
uninitialized.  So at the very least your check is going to yield random
results in that case.

What is it supposed to check for?  Note that memcpy_fromiovec() won't blow
up on NULL ->iov_base - with zero len it won't even look there and with
non-zero it'll fail with -EFAULT.

Was that intended to be if (unlikely(!len)) fail with EINVAL?  Something
entirely different?