udp: Fix a use-after-free in udp_multi_input()

"ip" is a pointer into the input mbuf chain, so we shouldn't access it
after the chain is freed.

Fix style at the call site while here.

Reported by: syzbot+7c8258509722af1b6145@syzkaller.appspotmail.com
Reviewed by: tuexen, glebius
Fixes: de2d47842e88 ("SMR protection for inpcbs")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33473