generic/470: Replace thin volume with blkdiscard -z

generic/470: Replace thin volume with blkdiscard -z

generic/470 was original designed to verify mmap(MAP_SYNC) which
is only valid to the DAX capable device(e.g. PMEM). Thin volume[1] was
introduced to fix the inconsistent filesystem issue[2] but it make
the test become not run because it doesn't support DAX. As Darrick
mentioned[3], zeroing the entire mapped range of scartch device
can fix the issue as well, so I try to use blkdiscard -z instead.

[1]: https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/commit/?id=fc5870da485aec0f9196a0f2bed32f73f6b2c664
[2]: https://lore.kernel.org/fstests/20190227061529.GF16436@dastard/
[3]: https://lore.kernel.org/linux-xfs/Y1NRNtToQTjs0Dbd@magnolia/T/#me0e77cb0ecd80bf4b5109e4433cb4863ae6e6727

Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>