arptables: Fix --proto-type mask formatting

arptables: Fix --proto-type mask formatting

Arptables accepts numeric --proto-type values and masks in any numeral
system identified by (absence of) prefix. Yet it prints the mask value
in hex without '0x'-prefix, breaking save and restore the same way
numeric --h-type output did.

In theory, this could be fixed either by adding the missing prefix or
printing the mask in decimal (like most other builtin matches do), but
since the value is printed in hex with prefix already, align mask output
with that.

Also a day 1 bug and consistent with legacy, so no Fixes: tag here as
well.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>