socket API: Use pool index instead of handle in sock_delete handler

The socket API used the socket index handle directly in the sock_delete handler,
resulting in "unknown client id" warnings, and a failure in return for socket clients.

Change-Id: Ia69f740ce0f834cd9b62b7157243a1f42bcad765
Signed-off-by: Ole Troan <ot@cisco.com>
diff --git a/src/vlibmemory/socket_api.c b/src/vlibmemory/socket_api.c
index 0f8c0f0..a8423d1 100644
--- a/src/vlibmemory/socket_api.c
+++ b/src/vlibmemory/socket_api.c
@@ -470,7 +470,7 @@
   if (!regp)
     return;
 
-  u32 reg_index = ntohl (mp->index);
+  u32 reg_index = socket_api_registration_handle_to_index (ntohl (mp->index));
   rp = vl_msg_api_alloc (sizeof (*rp));
   rp->_vl_msg_id = htons (VL_API_SOCKCLNT_DELETE_REPLY);
   rp->context = mp->context;