generic: test race between appending AIO DIO and fallocate

generic: test race between appending AIO DIO and fallocate

Dave Chinner reports[1] that an appending AIO DIO write to the second
block of a zero-length file and an fallocate request to the first block
of the same file can race to set isize, with the user-visible end result
that the file size is set incorrectly to one block long.  Write a small
test to reproduce the results.

[1] https://lore.kernel.org/linux-xfs/20191029100342.GA41131@bfoster/T/

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>