maple_tree: Fix check_find_2 testcase.

maple_tree: Fix check_find_2 testcase.

The check_find_2 exposed issues when the tree was a simple pointer.  The
pointer case was not being handled correctly in mas_start, or _mas_walk.
Fixing these two functions also required fixing calling functions to
check for MAS_ROOT (a root pointer).

There was also a bug in mas_next_entry where the next node was checked -
which is not possible if it's the root node.

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