xfs/614: determine the sector size of the fs image by doing a test format

xfs/614: determine the sector size of the fs image by doing a test format

In some cases (such as xfs always_cow=1), the configuration of the test
filesystem determines the sector size of the filesystem that we're going
to simulate formatting.  Concretely, even if TEST_DEV is a block device
with 512b sectors, the directio geometry can specify 4k writes to avoid
nasty RMW cycles.  When this happens, mkfs.xfs will set the sector size
to that 4k accordingly, but the golden output selection is wrong.  Fix
this.

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