block: check if zone_wplugs_hash exists in queue_zone_wplugs_show

Changhui reported a kernel crash when running this simple shell
reproducer:
 # cd /sys/kernel/debug/block && find  . -type f   -exec grep -aH . {} \;

The above results in a NULL pointer dereference if a device does not have
a zone_wplugs_hash allocated.

To fix this, return early if we don't have a zone_wplugs_hash.

Reported-by: Changhui Zhong <czhong@redhat.com>
Fixes: a98b05b ("block: Replace zone_wlock debugfs entry with zone_wplugs entry")
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/e5fec079dfca448cc21c425cfa5d7b291f5faa67.1714046443.git.johannes.thumshirn@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: morbidrsa

block/blk-zoned.c

@@ -1774,6 +1774,9 @@ int queue_zone_wplugs_show(void *data, struct seq_file *m)
 	unsigned int zwp_bio_list_size, i;
 	unsigned long flags;
 
+	if (!disk->zone_wplugs_hash)
+		return 0;
+
 	rcu_read_lock();
 	for (i = 0; i < disk_zone_wplugs_hash_size(disk); i++) {
 		hlist_for_each_entry_rcu(zwplug,