netfilter: arpt_mangle: fix return values of checkentry

In 135367b "netfilter: xtables: change xt_target.checkentry return type",
the type returned by checkentry was changed from boolean to int, but the
return values where not adjusted.

arptables: Input/output error

This broke arptables with the mangle target since it returns true
under success, which is interpreted by xtables as >0, thus
returning EIO.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>