ipsec: optimize SA outbound sequence update

For SA outbound packets, rte_atomic64_add_return is used to generate
SQN atomically. Use C11 atomics with RELAXED ordering for outbound SQN
update instead of rte_atomic ops which enforce unnecessary barriers on
aarch64.

Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>