maple_tree: Try harder to keep active node with mas_prev()

maple_tree: Try harder to keep active node with mas_prev()

Keep a reference to the node when possible with mas_prev().  This will
avoid re-walking the tree.  In keeping a reference to the node, keep the
last/index accurate to the range being referenced.  This means the limit
may be within the range, but the range may extend outside of the limit.

Also fix the single entry tree to respect the range (of 0), or set the
node to MAS_NONE in the case of shifting beyond 0.

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