ANDROID: mm/khugepaged: add missing vm_write_{begin|end}

Speculative page fault handler needs to detect concurrent pmd changes
and relies on vma seqcount for that. pmdp_collapse_flush(), set_huge_pmd() and collapse_and_free_pmd() can modify a pmd.
vm_write_{begin|end} are needed in the paths which can call these
functions for page fault handler to detect pmd changes.

Bug: 257443051
Change-Id: Ieb784b5f44901b66a594f61b9e7c91190ff97f80
Signed-off-by: Suren Baghdasaryan <surenb@google.com>