GFS2: Fix non-recursive truncate bug

Before this patch if you truncated a file to a smaller size it
wasn't freeing all the blocks properly. There are two reasons.

First, the metapath comparison was not comparing previous heights.
I added a function, mp_eq_to_hgt, which checks the metapath at
all heights prior to the target height.

Second, in function find_nonnull_ptr, it needed to zero out all
pointers for heights following the target height. Translated into
decimal integer terms, this way a number like 299, when incremented,
becomes 300, not 399. The 2 gets incremented to 3, and the following
digits need to be reset.

These two things allow the truncate state machine to properly find
the blocks it needs to delete.

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