mm: ashmem: Make sure vma is valid while flushing

Ensure that the vma that the ashmem region is tied to
is valid before flushing and hold the mm semaphore so
that the vma does not get destroyed from under while
it's being flushed.

Change-Id: I5216aa3a4fa983f97909c1132d12abe777f60530
CRs-Fixed: 326685
Orig-Change-Id: I86320ffe9e6570e6ffa7dda68ab26be7aed86afd
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>
(cherry picked from commit bd887f9df9de92dbbea842213783bc6a7f7d7d5e)