SUNRPC: 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);
// </smpl>
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/sched.c

@@ -1092,13 +1092,9 @@ void
 rpc_destroy_mempool(void)
 {
 	rpciod_stop();
-	if (rpc_buffer_mempool)
 	mempool_destroy(rpc_buffer_mempool);
-	if (rpc_task_mempool)
 	mempool_destroy(rpc_task_mempool);
-	if (rpc_task_slabp)
 	kmem_cache_destroy(rpc_task_slabp);
-	if (rpc_buffer_slabp)
 	kmem_cache_destroy(rpc_buffer_slabp);
 	rpc_destroy_wait_queue(&delay_queue);
 }