sys/dev/irdma/irdma_cm: fix compile without INET

irdma_get_vlan_ipv4() calls ip_ifp_find() even if INET isn't defined, in
which case this function isn't available.

Stub this out for the non-INET case to return an error (0xffff) instead.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1156

(cherry picked from commit 0478a0356272ab3907af9d97711f0bb8b14bf38b)
(cherry picked from commit 7aba922879474189e5694ffb92a51ab387ad3dd6)
Approved-by: re (cperciva)