io_uring/zcrx: convert to use netmem_desc

io_uring/zcrx: convert to use netmem_desc

Convert zcrx to struct netmem_desc, and use struct net_iov::desc to
access its fields instead of struct net_iov inner union alises.
zcrx only directly reads niov->pp, so with this patch it doesn't depend
on the union anymore.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Byungchul Park <byungchul@sk.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>