xfs: fix stale disk exposure after crash

These two patches try to shrink the window during which a crash during
writeback can expose stale disk contents.  The first patch causes
delalloc reservations to be converted to unwritten extents for any
writeback that's going on within EOF.  The second patch forces data
integrity flushes for any range of a file that's beyond the on-disk EOF
to flush everything from the on-disk EOF to the end of the originally
specified range.  This doesn't fix all the problems, but it gets us
closer.