ipfw: fix lookup dst-ip opcode

Opcode handling should not fall through to the LOOKUP_DSCP type.

Reviewed by: melifaro
Obtained from: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D53775

(cherry picked from commit 8012c61bef3bb19a48d8459b38b65e27d46c186c)