Use pru_send function to check socket splicing compatibility. Only

Use pru_send function to check socket splicing compatibility.  Only
checking socket type is not sufficient as it could splice together
unix and inet sockets resulting in crashes.  As splicing is about
sending, the same send function looks like a good criteria.
Reported-by: syzbot+fc6901d63d858d5dd00a@syzkaller.appspotmail.com
Reported-by: syzbot+0e026f1bf8b259c6395e@syzkaller.appspotmail.com
OK gnezdo@