sunrpc: fix crash in rpc_malloc()

While the comment says:
 * To prevent rpciod from hanging, this allocator never sleeps,
 * returning NULL if the request cannot be serviced immediately.

The function does not actually check for NULL pointers being returned.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/sched.c

@@ -767,6 +767,10 @@ void *rpc_malloc(struct rpc_task *task, size_t size)
 		buf = mempool_alloc(rpc_buffer_mempool, gfp);
 	else
 		buf = kmalloc(size, gfp);
+
+	if (!buf)
+		return NULL;
+
 	buf->len = size;
 	dprintk("RPC: %5u allocated buffer of size %zu at %p\n",
 			task->tk_pid, size, buf);