generic: add a new test for racing AIO COW writes

generic: add a new test for racing AIO COW writes

This can be used to trigger an assert in the current XFS code
because it can't handle the case where there are COW extents on a
file, but none at or below the range converted by the AIO completion
handler.

Note that it doesn't trigger the assert 100% but fairly reliably.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>