problem.

In ext3_new_block() we increment i_blocks early, so the
quota operation can be performed outside lock_super().
But if the block allocation ends up failing, we forget to
undo the allocation.  

This is not a serious bug, and probably does not warrant
an upgrade for production machines.  Its effects are:

1) errors are generated from e2fsck and

2) users could appear to be over quota when they really aren't.

The patch undoes the accounting operation if the allocation
ends up failing.