bdev: Refresh bdev size for disks without partitioning

Currently, block device size in not updated on second and further open
for block devices where partition scan is disabled. This is particularly
annoying for example for DVD drives as that means block device size does
not get updated once the media is inserted into a drive if the device is
already open when inserting the media. This is actually always the case
for example when pktcdvd is in use.

Fix the problem by revalidating block device size on every open even for
devices with partition scan disabled.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>