Changes

omo_io: poll: Support multiple iov on all read/write variants

The logic was already in place for io_uring, but was missing for poll
backend. So this patch mainly takes the existing logic from there and
adapts it to poll.
This way we can get some easy optimizations even when using poll(),
for programs doing multiple msgb sends on the same event loop iteration.

Related: OS#6705
Change-Id: I64a2a6725f91b5e5f575c6fc104bd58b50063873