strparser: Call sock_owned_by_user_nocheck

strparser wants to check socket ownership without producing any
warnings. As indicated by the comment in the code, it is permissible
for owned_by_user to return true.

Fixes: 43a0c675

 ("strparser: Stream parser for messages")
Reported-by: syzbot <syzkaller@googlegroups.com>
Reported-and-tested-by:  <syzbot+c91c53af67f9ebe599a337d2e70950366153b295@syzkaller.appspotmail.com>
Signed-off-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>