damon: Convert __damon_va_three_regions to use the VMA iterator

damon: Convert __damon_va_three_regions to use the VMA iterator

This rather specialised walk can use the VMA iterator.  If this proves
to be too slow, we can write a custom routine to find the two largest
gaps, but it will be somewhat complicated, so let's see if we need it
first.

Update the kunit test case to use the maple tree.  This also fixes an
issue with the kunit testcase not adding the last VMA to the list.

Fixes: 17ccae8bb5c9 (mm/damon: add kunit tests)
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>