xfs/259: try to force loop device block size

xfs/259: try to force loop device block size

Starting with 6.15-rc1, loop devices created with directio mode enabled
will set their logical block size to whatever STATX_DIO_ALIGN on the
host filesystem reports.  If you want fstests to set up loop devices in
directio mode and TEST_DEV is a block device with 4k sectors, this will
cause conflicts with this test's usage of mkfs with different block
sizes.  Enhance the existing _create_loop_device so that tests can force
the loop device block size to 512 bytes, which is implied by scenarios
such as "device size is 4T - 2048 bytes".

Also fix xfs/078 which simply needs the blocksize to be set.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>