ipv6: icmp: use this_cpu_read() in icmpv6_sk()

In general, this_cpu_read(*X) is faster than *this_cpu_ptr(X)

Also remove the inline attibute, totally useless.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>