netfilter: xtables: check for standard verdicts in policies

This adds the second check that Rusty wanted to have a long time ago. :-)

Base chain policies must have absolute verdicts that cease processing
in the table, otherwise rule execution may continue in an unexpected
spurious fashion (e.g. next chain that follows in memory).

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>