There's a case when an indirect block gets dirtied for no good
reason - when there's a hole starting in the middle of area
covered by it and spanning past its end, and truncate() is done
precisely to the beginning of the hole.

The block is obviously not modified at all - all removals happen
beyond it.  However, existing code ends up dirtying it just in
case.  It's trivial to fix and while it's not a real bug by any
stretch of imagination, it makes the damn thing harder to follow.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>