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 is not necessary.  Expand or contract
the existing VMA instead.  This avoids unnecessary tree manipulations
and allocations.

Once the VMA is known, use it directly when populating to avoid
unnecessary lookup work.

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