in6: Consolidate IN6_ARE_ADDR_EQUAL definitions

It is ok to use memcmp() in the kernel. No functional change intended.

Reviewed by: glebius, melifaro
MFC after: 1 week
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37028