vlan: slight optimization for vlan_do_receive()

vlan: slight optimization for vlan_do_receive()

According Joe's suggestion, maybe it'd be faster to add an unlikely to
the test for PCKET_OTHERHOST, so I add it and see whether the performance
could be better, although the differences is so small and negligible, but
it is hard to catch that any lower device would set the skb type to
PACKET_OTHERHOST, so most of time, I think it make sense to add unlikely
for the test.

Cc: Joe Perches <joe@perches.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>