maple_tree: Update unmapped area name, remove mas->node == NULL

maple_tree: Update unmapped area name, remove mas->node == NULL
possibility, 3 way split fixes, overflow slot fix, calc split fix,
altered descend adopt algorithm.

Many changes.

1. unmapped area => empty area
2. mas->node can no longer be null
3. splitting into 3 nodes can occur at non-leaf nodes.
4. calc split could potentially create an overflow or a deficient node.
5. drop Private from comments
6. comment fixes
7. rename combine_separate to mas_spanning_rebalance

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