[NETFILTER]: ip_tables: ipt and ipt_compat checks unification

Matches and targets verification is duplicated in normal and compat processing
ways. This patch refactors code in order to remove this.

Signed-off-by: Dmitry Mishin <dim@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>