gfs2: Improve non-recursive delete algorithm

In rare cases, the current non-recursive delete algorithm doesn't
deallocate empty intermediary indirect blocks.  This should have very
little practical effect, but deallocating all blocks correctly should
still be preferable as it is cleaner and easier to validate.

The fix consists of using the first block to deallocate to compute the
start marker of the truncate point instead of the last block that needs
to be kept.  With that change, computing which indirect blocks are still
needed becomes relatively easy.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>