nft: Merge nft_*_rule_find() functions

Both ebtables and arptables are fine with using nft_ipv46_rule_find()
instead of their own implementations. Take the chance and move the
former into nft.c as a static helper since it is used in a single place,
only. Then get rid of the callback from family_ops.

Signed-off-by: Phil Sutter <phil@nwl.cc>