UBI: block: fix dereference on uninitialized dev

commit 4df38926 ("UBI: block: Avoid disk size integer overflow")
introduced a dereference on dev (which is not initialized at that
point) when printing a warning message.  Re-order disk_capacity check
after the dev is found.

Found by cppcheck:
 [drivers/mtd/ubi/block.c:509]: (error) Uninitialized variable: dev

Artem: tweak the error message a bit
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Acked-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/block.c

@@ -504,11 +504,6 @@ static int ubiblock_resize(struct ubi_volume_info *vi)
 	struct ubiblock *dev;
 	u64 disk_capacity = ((u64)vi->size * vi->usable_leb_size) >> 9;
 
-	if ((sector_t)disk_capacity != disk_capacity) {
-		ubi_warn("%s: the volume is too big, cannot resize (%d LEBs)",
-			 dev->gd->disk_name, vi->size);
-		return -EFBIG;
-	}
 	/*
 	 * Need to lock the device list until we stop using the device,
 	 * otherwise the device struct might get released in
@@ -520,6 +515,12 @@ static int ubiblock_resize(struct ubi_volume_info *vi)
 		mutex_unlock(&devices_mutex);
 		return -ENODEV;
 	}
+	if ((sector_t)disk_capacity != disk_capacity) {
+		mutex_unlock(&devices_mutex);
+		ubi_warn("%s: the volume is too big (%d LEBs), cannot resize",
+			 dev->gd->disk_name, vi->size);
+		return -EFBIG;
+	}
 
 	mutex_lock(&dev->dev_mutex);
 	set_capacity(dev->gd, disk_capacity);