netfilter: arp_tables: fix invoking 32bit "iptable -P INPUT ACCEPT" failed in 64bit kernel

Since 09d96860 ("netfilter: x_tables: do compat validation via
translate_table"), it used compatr structure to assign newinfo
structure.  In translate_compat_table of ip_tables.c and ip6_tables.c,
it used compatr->hook_entry to replace info->hook_entry and
compatr->underflow to replace info->underflow, but not do the same
replacement in arp_tables.c.

It caused invoking 32-bit "arptbale -P INPUT ACCEPT" failed in 64bit
kernel.
--------------------------------------
root@qemux86-64:~# arptables -P INPUT ACCEPT
root@qemux86-64:~# arptables -P INPUT ACCEPT
ERROR: Policy for `INPUT' offset 448 != underflow 0
arptables: Incompatible with this kernel
--------------------------------------

Fixes: 09d96860 ("netfilter: x_tables: do compat validation via translate_table")
Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>