mm: conditionally write-lock VMA in free_pgtables

mm: conditionally write-lock VMA in free_pgtables

Normally free_pgtables needs to lock affected VMAs except for the case
when VMAs were isolated under VMA write-lock. munmap() does just that,
isolating while holding appropriate locks and then downgrading mmap_lock
and dropping per-VMA locks before freeing page tables.
Add a parameter to free_pgtables and unmap_region for such scenario.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>