ipfw(8) crashes when ext6hdr option is omitted

Verify that the option is passed, error out if it's not.
The problem can be trivially triggered with ipfw add allow ext6hdr.

PR: 253169
Reviewed by: kp@
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28447

(cherry picked from commit 682c31db4ecfb8fc6cac0e8ad4945c03379ea3d1)