maple_tree: Introduce mas_prev_slot() interface

maple_tree: Introduce mas_prev_slot() interface

Sometimes the user needs to revert to the previous slot, regardless of
if it is empty or not.  Add an interface to go to the previous slot.

Since there can't be two consecutive NULLs in the tree, the mas_prev()
function can be implemented by calling mas_prev_slot() a maximum of 2
times.  Change the underlying interface to use mas_prev_slot() to align
the code.

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