[IPSEC] flow: reorder "struct flow_cache_entry" and remove SLAB_HWCACHE_ALIGN

1) We can shrink sizeof(struct flow_cache_entry) by 8 bytes on 64bit arches.
2) No need to align these structures to hardware cache lines, this only waste 
   ram for very litle gain.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>