generic/032: add xfs unwritten extent data corruption reproducer

generic/032: add xfs unwritten extent data corruption reproducer

XFS had a data corruption problem where writeback of pages to unwritten
extents would fail to run unwritten extent conversion at I/O completion.
This causes subsequent reads of written, but unconverted regions to
return zeroes. This occurs on sub-page block size filesystems when
writeback contends for the inode lock (e.g., with a file writer).

Add a test that creates the conditions to reproduce the data corruption
and detect it by looking for unwritten extents after all said extents
have been overwritten.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>