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>