pfctl: fix FOM_ICMP/POM_STICKYADDRESS clash

pass inet proto icmp icmp-type {unreach}
pass route-to (if0 127.0.0.1/8) sticky-address inet

The wrong struct was being tested. The parser tries to prevent
"sticky-address sticky-address" syntax but was actually cross-rule
enforcing that ICMP filter cannot be before the use of "sticky-address"
in next rule.

MFC after: 2 weeks
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D36050