ipsec: move the IPSec SA pool out of ipsec_main

Type: refactor

this allows the ipsec_sa_get funtion to be moved from ipsec.h to
ipsec_sa.h where it belongs.
Also use ipsec_sa_get throughout the code base.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I2dce726c4f7052b5507dd8dcfead0ed5604357df
diff --git a/src/vnet/ipsec/ipsec_input.c b/src/vnet/ipsec/ipsec_input.c
index 2ebc6c5..15a0796 100644
--- a/src/vnet/ipsec/ipsec_input.c
+++ b/src/vnet/ipsec/ipsec_input.c
@@ -111,7 +111,7 @@
   vec_foreach (i, spd->policies[IPSEC_SPD_POLICY_IP4_INBOUND_PROTECT])
   {
     p = pool_elt_at_index (im->policies, *i);
-    s = pool_elt_at_index (im->sad, p->sa_index);
+    s = ipsec_sa_get (p->sa_index);
 
     if (spi != s->spi)
       continue;
@@ -167,7 +167,7 @@
   vec_foreach (i, spd->policies[IPSEC_SPD_POLICY_IP6_INBOUND_PROTECT])
   {
     p = pool_elt_at_index (im->policies, *i);
-    s = pool_elt_at_index (im->sad, p->sa_index);
+    s = ipsec_sa_get (p->sa_index);
 
     if (spi != s->spi)
       continue;