LinuxKPI: add ether_addr_equal_unaligned()

Replace the implementation for ether_addr_equal() with
ether_addr_equal_unaligned() and add a define for ether_addr_equal()
pointing to the now ether_addr_equal_unaligned() implementation.
This way ether_addr_equal_unaligned() cannot be broken by accident [1].

Suggested by: emaste [1]
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30425