xfs/259: drop the 512-byte fsblock logic from this test

xfs/259: drop the 512-byte fsblock logic from this test

V5 filesystems do not support 512-byte fsblocks, and mkfs.xfs has long
defaulted to V5 filesystems.  Drop the 512 from the test loops, which
means we can get rid of all the _fs_has_crcs logic.  As a further
cleanup, use the truncate -s command to create the sparse file instead
of dd since even RHEL7 supports the -s switch.

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