ubi: block: fix null-pointer-dereference in ubiblock_create()

Similar to commit adbf4c4954e3 ("ubi: block: fix memleak in
ubiblock_create()"), 'dev->gd' is not assigned but dereferenced if
blk_mq_alloc_tag_set() fails, and leading to a null-pointer-dereference.
Fix it by using pr_err() and variable 'dev' to print error log.

Additionally, the log in the error handle path of idr_alloc() has
been improved by using pr_err(), too. Before initializing device
name, using dev_err() will print error log with 'null' instead of
the actual device name, like this:
block (null): ...
~~~~~~
It is unclear. Using pr_err() can print more details of the device.
The improved log is:
ubiblock0_0: ...

Fixes: 77567b25ab9f ("ubi: use blk_mq_alloc_disk and blk_cleanup_disk")
Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Li Nan <[e-mail]>
Reviewed-by: Zhihao Cheng <[e-mail]>
Reviewed-by: Daniel Golle <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

drivers/mtd/ubi/block.c

···
 
 	ret = blk_mq_alloc_tag_set(&dev->tag_set);
 	if (ret) {
-		dev_err(disk_to_dev(dev->gd), "blk_mq_alloc_tag_set failed");
+		pr_err("ubiblock%d_%d: blk_mq_alloc_tag_set failed\n",
+			dev->ubi_num, dev->vol_id);
 		goto out_free_dev;
 	}
 
···
 	gd->minors = 1;
 	gd->first_minor = idr_alloc(&ubiblock_minor_idr, dev, 0, 0, GFP_KERNEL);
 	if (gd->first_minor < 0) {
-		dev_err(disk_to_dev(gd),
-			"block: dynamic minor allocation failed");
+		pr_err("ubiblock%d_%d: block: dynamic minor allocation failed\n",
+			dev->ubi_num, dev->vol_id);
 		ret = -ENODEV;
 		goto out_cleanup_disk;
 	}