netfilter: nft_masq: deduplicate eval call-backs

nft_masq has separate ipv4 and ipv6 call-backs which share much of their
code, and an inet one switch containing a switch that calls one of the
others based on the family of the packet.  Merge the ipv4 and ipv6 ones
into the inet one in order to get rid of the duplicate code.

Const-qualify the `priv` pointer since we don't need to write through it.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Florian Westphal <fw@strlen.de>