dio_writeback_race: align the directio buffer to base page size

dio_writeback_race: align the directio buffer to base page size

There's no need to align the memory buffer for the direcio write to the
file allocation unit size; base page size will do.  This fixes the
following error when generic/761 is run against an XFS rt filesystem
with a 28k rtextsize:

 QA output created by 761
+failed to allocate aligned memory
+cat: /opt/foobar: No such file or directory
 Silence is golden

Cc: <fstests@vger.kernel.org> # v2025.02.16
Fixes: 17fb49493426ad ("fstests: add a generic test to verify direct IO writes with buffer contents change")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>