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.

Move some limit & flag verifications out of the do_brk_flags() function
to use only relevant checks in the code path of bkr() and
vm_brk_flags().

Set the vma to check if it can expand in vm_brk_flags() if extra
criteria are met.

Drop userfaultfd from do_brk_flags() path and only use it in
vm_brk_flags() path since that is the only place a munmap will happen.

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