xtables: fix missing ipt_entry for MASQUERADE target

xtables: fix missing ipt_entry for MASQUERADE target

The MASQUERADE target relies on the ipt_entry information that is
set in ->post_parse, which is too late.

Add a new hook called ->pre_parse, that sets the protocol
information accordingly.

Thus:

	xtables -4 -A POSTROUTING -t nat -p tcp \
		-j MASQUERADE --to-ports 1024

works again.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>