mm/mmap: Change do_brk_flags() to expand existing VMA and add

mm/mmap: Change do_brk_flags() to expand existing VMA and add
do_brk_munmap()

Avoid allocating a new VMA when it a vma modification can occur.  When a
brk() can expand or contract a VMA, then the single store operation will
only modify one index of the maple tree instead of causing a node to
split or coalesce.  This avoids unnecessary allocations/frees of maple
tree nodes and VMAs.

Use the advanced API for the maple tree to avoid unnecessary walks of
the tree.

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