arp: postpone addr_type calculation to as late as possible

The addr_type retrieval can be costly, so it's worth trying to avoid its
calculation as much as possible. This patch makes it calculated only
for gratuitous ARP packets. This is especially important since later we
may want to move is_garp calculation outside of arp_accept block, at
which point the costly operation will be executed for all setups.

The patch is the result of a discussion in net-dev:
http://marc.info/?l=linux-netdev&m=149506354216994



Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Ihar Hrachyshka <ihrachys@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>