mm/khugepaged: write-lock VMA while collapsing a huge page

mm/khugepaged: write-lock VMA while collapsing a huge page

Protect VMA from concurrent page fault handler while collapsing a huge
page. Page fault handler needs a stable PMD to use PTL and relies on
per-VMA lock to prevent concurrent PMD changes. pmdp_collapse_flush(),
set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will
not be detected by a page fault handler without proper locking.

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