aio-dio-append-write-fallocate-race: fix directio buffer alignment bugs

aio-dio-append-write-fallocate-race: fix directio buffer alignment bugs

This program fails on filesystems where the stat() block size isn't a
strict power of two because it foolishly feeds that to posix_memalign to
allocate an aligned memory buffer for directio.  posix_memalign requires
the alignment value to be a power of two, so generic/586 fails.

The system page size generally works well for directio buffers, so use
that instead.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>