ipv6: sr: static percpu allocation for hmac_ring

Current allocations are not NUMA aware, and lack proper
cleanup in case of error.

It is perfectly fine to use static per cpu allocations for 256 bytes
per cpu.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Lebrun <david.lebrun@uclouvain.be>
Acked-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>