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.
Addresses-Coverity-ID: 1496811 ("Logically dead code")
Fixes: 921b68692abb ("xsk: Enable sharing of dma mappings")
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>