ensure lookup was successful before suppressing it
fib6_rules: make error handling consistent with IPv4

This compile-only-tested patch would make the error reporting consistent
with its ipv4 counterpart. arg->result should only be NULL iff the
function returns -EAGAIN. So we are clean here.

(It seems this patch also fixes a potential nullptr dereference.)
fib_rules/fib6_rules: decrement refcount if suppressing
fib_rules: use bool return values