netfilter: x_tables: print correct hook names for ARP

arptables 0.0.4 (released on 10th Jan 2013) supports calling the
CLASSIFY target, but on adding a rule to the wrong chain, the
diagnostic is as follows:

	# arptables -A INPUT -j CLASSIFY --set-class 0:0
	arptables: Invalid argument
	# dmesg | tail -n1
	x_tables: arp_tables: CLASSIFY target: used from hooks
	PREROUTING, but only usable from INPUT/FORWARD

This is incorrect, since xt_CLASSIFY.c does specify
(1 << NF_ARP_OUT) | (1 << NF_ARP_FORWARD).

This patch corrects the x_tables diagnostic message to print the
proper hook names for the NFPROTO_ARP case.

Affects all kernels down to and including v2.6.31.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>