io_uring/net: fix iter retargeting for selected buf

When using selected buffer feature, io_uring delays data iter setup
until later. If io_setup_async_msg() is called before that it might see
not correctly setup iterator. Pre-init nr_segs and judge from its state
whether we repointing.

Cc: stable@vger.kernel.org
Reported-by:  <syzbot+a4c6e5ef999b68b26ed1@syzkaller.appspotmail.com>
Fixes: 0455d4cc

 ("io_uring: add POLL_FIRST support for send/sendmsg and recv/recvmsg")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0000000000002770be06053c7757@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>