IPSEC: move SA counters into the stats segment

IPSEC: move SA counters into the stats segment

1) stats are accessed via the stat segment which is more condusive to
   monitoring
2) stats are accurate in the presence of multiple threads. There's no
   guarantee that an SA is access from only one worker.

Change-Id: Id5e217ea253ddfc9480aaedb0d008dea031b1148
Signed-off-by: Neale Ranns <nranns@cisco.com>