xsk: Fix null check on error return path

Currently, dma_map is being checked, when the right object identifier
to be null-checked is dma_map->dma_pages, instead.

Fix this by null-checking dma_map->dma_pages.

Fixes: 921b6869 ("xsk: Enable sharing of dma mappings")
Addresses-Coverity-ID: 1496811 ("Logically dead code")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20200902150750.GA7257@embeddedor

net/xdp/xsk_buff_pool.c

@@ -287,7 +287,7 @@ static struct xsk_dma_map *xp_create_dma_map(struct device *dev, struct net_devi
 		return NULL;
 
 	dma_map->dma_pages = kvcalloc(nr_pages, sizeof(*dma_map->dma_pages), GFP_KERNEL);
-	if (!dma_map) {
+	if (!dma_map->dma_pages) {
 		kfree(dma_map);
 		return NULL;
 	}