maple_tree: Fix finding the end of a node when the node ends in a NULL.

maple_tree: Fix finding the end of a node when the node ends in a NULL.
Also fix the limit calculation on gaps when searches backwards goes into
adjacent nodes during the search.

Ignoring the end null may cause gaps to be skipped when searching
backwards.

Adjacent nodes caused the max limit to be reset to the parent but never
set if the very next entry also had a potential gap that would work.

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