icmp6: rate limit our echo replies

The generation of ICMP6_ECHO_REPLY bypasses icmp6_error(), thus rate
limit was not applied.

Reviewed by: tuexen, zlei
Differential Revision: https://reviews.freebsd.org/D44480

(cherry picked from commit 32aeee8ce7e72738fff236ccd5629d55035458f8)