netfilter: ebt: Use new helper ebt_invalid_target to check target

Use the new helper function ebt_invalid_target instead of the old
macro INVALID_TARGET and other duplicated codes to enhance the readability.

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>