icmp6: make icmp6_ratelimit() responsible to update the stats counter

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D44479
Approved by: re (cperciva)

(cherry picked from commit c6c96aaba8dd74eb39469ed156ff19cc31d599b7)
(cherry picked from commit dc167e64dce3ffc212e4f7a2268587abcc22c036)