xdp: add NULL pointer check in __xdp_return()

rhashtable_lookup() can return NULL. so that NULL pointer
check routine should be added.

Fixes: 02b55e56 ("xdp: add MEM_TYPE_ZERO_COPY")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/xdp.c

@@ -345,6 +345,7 @@ static void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
 		rcu_read_lock();
 		/* mem->id is valid, checked in xdp_rxq_info_reg_mem_model() */
 		xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
+		if (!WARN_ON_ONCE(!xa))
 			xa->zc_alloc->free(xa->zc_alloc, handle);
 		rcu_read_unlock();
 	default: