Revert "Revert "mm/mmap: regression fix for unmapped_area{_topdown}""

This reverts commit 52ace503.
The issue that required the revert is fixed by:
0257d990 ("maple_tree: make maple state reusable after mas_empty_area()")

Bug: 281094761
Change-Id: I97b45525689097d0c1369f81a994d50f0662c9c2
Signed-off-by: Suren Baghdasaryan <surenb@google.com>