mm/mmap: Use maple tree for unmapped_area{_topdown}

mm/mmap: Use maple tree for unmapped_area{_topdown}

The maple tree code was added to find the unmapped area in a previous
commit and was checked against what the rbtree returned, but the actual
result was never used.  Start using the maple tree implementation and
remove the rbtree code.

Add kernel documentation comment for these functions.

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