fix spurious error setting SA on ipsec intf

Reverse the polarity on test to determine if old SA session
deletion succeeded. 0 == success, not failure.

Change-Id: I499cb04c7f13165e6c92367d4385057b77fe3836
Signed-off-by: Matthew Smith <mgsmith@netgate.com>
diff --git a/src/vnet/ipsec/ipsec_if.c b/src/vnet/ipsec/ipsec_if.c
index 3054af1..e8b1a4e 100644
--- a/src/vnet/ipsec/ipsec_if.c
+++ b/src/vnet/ipsec/ipsec_if.c
@@ -586,7 +586,7 @@
   if (ipsec_get_sa_index_by_sa_id (old_sa->id) == old_sa_index)
     hash_unset (im->sa_index_by_sa_id, old_sa->id);
 
-  if (!ipsec_add_del_sa_sess_cb (im, old_sa_index, 0))
+  if (ipsec_add_del_sa_sess_cb (im, old_sa_index, 0))
     {
       clib_warning ("IPsec backend add/del callback returned error");
       return VNET_API_ERROR_SYSCALL_ERROR_1;