block: Invalidate the cache for a parent block-device if blkdev_issue_flush()
was called for a partition

Executing a BLKFLSBUF-ioctl on a partition flushes the caches for that
partition but reading data through the parent device will still return
the old cached data.

The cache for the block-device is not synced if the block-device is kept
open (due to a mounted partition, for example). Only when all users for
the disk have exited, the cache for the disk is made consistent again.

Calling invalidate_bdev() on the parent block-device in case
blkdev_issue_flush() was called for a partition fixes this.

The problem can be worked around by forcing the caches to be flushed
with either
	# blockdev --flushbufs ${dev_disk}
or
	# echo 3 > /proc/sys/vm/drop_caches
CC: Bryn M. Reeves <bmr@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>