maple_tree: Introduce mas_next_slot() interface

maple_tree: Introduce mas_next_slot() interface

Sometimes, during a tree walk, the user needs the next slot regardless
of if it is empty or not.  Add an interface to get the next slot.

Since there are no consecutive NULLs allowed in the tree, the mas_next()
function can only advance two slots at most.  So use the new
mas_next_slot() interface to align both implementations.  Use this
method for mas_find() as well.

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