net: core: drop null test before destroy functions

Remove unneeded NULL test.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@ expression x; @@
-if (x != NULL) {
  \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
  x = NULL;
-}
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -2740,10 +2740,8 @@ static void req_prot_cleanup(struct request_sock_ops *rsk_prot)
 		return;
 	kfree(rsk_prot->slab_name);
 	rsk_prot->slab_name = NULL;
-	if (rsk_prot->slab) {
 	kmem_cache_destroy(rsk_prot->slab);
 	rsk_prot->slab = NULL;
-	}
 }
 
 static int req_prot_init(const struct proto *prot)
@@ -2828,10 +2826,8 @@ void proto_unregister(struct proto *prot)
 	list_del(&prot->node);
 	mutex_unlock(&proto_list_mutex);
 
-	if (prot->slab != NULL) {
 	kmem_cache_destroy(prot->slab);
 	prot->slab = NULL;
-	}
 
 	req_prot_cleanup(prot->rsk_prot);