There are code duplications of a masked ethernet address comparison here
so make it a separate function instead.

Miscellanea:

o Neaten alignment of FWINV macro uses to make it clearer for the reader
Signed-off-by: NJoe Perches <joe@perches.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>