Make the test suite work for devices with a 4096 byte block size

Direct I/O reads and writes with a size of 512 bytes trigger an
I/O error on devices with a sector size of 4096 bytes. Hence change
the block size from 512 into 4096.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Lee Duncan <lduncan@suse.com>