blk-mq: fix null pointer dereference in blk_mq_clear_rq_mapping()

Our syzkaller report a null pointer dereference, root cause is
following:

__blk_mq_alloc_map_and_rqs
 set->tags[hctx_idx] = blk_mq_alloc_map_and_rqs
  blk_mq_alloc_map_and_rqs
   blk_mq_alloc_rqs
    // failed due to oom
    alloc_pages_node
    // set->tags[hctx_idx] is still NULL
    blk_mq_free_rqs
     drv_tags = set->tags[hctx_idx];
     // null pointer dereference is triggered
     blk_mq_clear_rq_mapping(drv_tags, ...)

This is because commit 63064be1 ("blk-mq:
Add blk_mq_alloc_map_and_rqs()") merged the two steps:

1) set->tags[hctx_idx] = blk_mq_alloc_rq_map()
2) blk_mq_alloc_rqs(..., set->tags[hctx_idx])

into one step:

set->tags[hctx_idx] = blk_mq_alloc_map_and_rqs()

Since tags is not initialized yet in this case, fix the problem by
checking if tags is NULL pointer in blk_mq_clear_rq_mapping().

Fixes: 63064be1

 ("blk-mq: Add blk_mq_alloc_map_and_rqs()")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20221011142253.4015966-1-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>