ip6_mroute: Make MF6CFIND a regular function

This is more natural and corresponds more closely to the v4 multicast
routing code. No functional change intended.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54983

(cherry picked from commit b370fcc716b9cfd4d08e291f0009f02452c84d64)