drivers/lightnvm: fix a null-ptr-deref bug in pblk-core.c

At line 294 in drivers/lightnvm/pblk-write.c, function pblk_gen_run_ws is
called with actual param GFP_ATOMIC. pblk_gen_run_ws call mempool_alloc
using "GFP_ATOMIC" flag, so mempool_alloc can return null. So we need to
check the return-val of mempool_alloc to avoid null-ptr-deref bug.

Signed-off-by: tangzhenhao <tzh18@mails.tsinghua.edu.cn>
Reviewed-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/pblk-core.c

@@ -1869,6 +1869,10 @@ void pblk_gen_run_ws(struct pblk *pblk, struct pblk_line *line, void *priv,
 	struct pblk_line_ws *line_ws;
 
 	line_ws = mempool_alloc(&pblk->gen_ws_pool, gfp_mask);
+	if (!line_ws) {
+		pblk_err(pblk, "pblk: could not allocate memory\n");
+		return;
+	}
 
 	line_ws->pblk = pblk;
 	line_ws->line = line;