[ARP]: Minus one level of indentation in arp_req_set

The ATF_PUBL requests are handled completely separate from
the others. Emphasize it with a separate function. This also
reduces the indentation level.

The same issue exists with the arp_delete_request, but
when I tried to make it in one patch diff produced completely
unreadable patch. So I split it into two, but they may be
done with one commit.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>