Evaluate packet size after the firewall had its chance in the ip6 fast path

Defer the packet size check until after the firewall has had a look at it. This
means that the firewall now has the opportunity to (re-)fragment an oversized
packet.
This mirrors what the slow path does.

Reviewed by: ae
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D12779