pfctl: fix FOM_ICMP/POM_STICKYADDRESS clash

[• franco_opnsense.org]

Summary

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.