ubi: Check for too small LEB size in VTBL code

ubi: Check for too small LEB size in VTBL code

If the LEB size is smaller than a volume table record we cannot
have volumes.
In this case abort attaching.

Cc: Chenyuan Yang <cy54@illinois.edu>
Cc: stable@vger.kernel.org
Fixes: 801c135ce73d ("UBI: Unsorted Block Images")
Reported-by: Chenyuan Yang <cy54@illinois.edu>
Closes: https://lore.kernel.org/linux-mtd/1433EB7A-FC89-47D6-8F47-23BE41B263B3@illinois.edu/
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>