io_uring/net: abstract out io_send_import() helper

io_uring/net: abstract out io_send_import() helper

If provided buffers are used, this helper can be used to import the
necessary data from a provided buffer group. Only one user so far, but
add it in preparation of adding another one. While doing so, also split
the actual import into an iov_iter out into a separate helper.

In preparation for needing to know the number of mapped segments, return
that instead. It still returns < 0 on error.

Signed-off-by: Jens Axboe <axboe@kernel.dk>