maple_tree: Fix data_end calculation and root expansion.

maple_tree: Fix data_end calculation and root expansion.

Root expansion needs to set the pivot to the empty value if there is no
zero for the append to function correctly.

In append, we know the data end will always be at least 1, so take two
off the idx when checking the previous pivot value (since we also add 1
to the end for the new termination).

Append will need to be altered to cover off the overflow possibility in
the future.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>