net: save some cycles when doing skb_attempt_defer_free()

Normally, we don't face these two exceptions very often meanwhile
we have some chance to meet the condition where the current cpu id
is the same as skb->alloc_cpu.

One simple test that can help us see the frequency of this statement
'cpu == raw_smp_processor_id()':
1. running iperf -s and iperf -c [ip] -P [MAX CPU]
2. using BPF to capture skb_attempt_defer_free()

I can see around 4% chance that happens to satisfy the statement.
So moving this statement at the beginning can save some cycles in
most cases.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>