generic: test stale data exposure after writeback crash

generic: test stale data exposure after writeback crash

XFS has historically had a stale data exposure window if a crash
occurs after a delalloc->physical extent conversion but before
writeback completes to the associated extent. While this should be a
rare occurrence in production environments due to typical writeback
ordering and such, it is not guaranteed in all cases until data
extents are initialized as unwritten (or otherwise zeroed) before
they are written.

Add a test that performs selective writeback ordering to reproduce
stale data exposure after a crash. Note that this test currently
fails on XFS.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>