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.

CRs-Fixed: 326685
Change-Id: I86320ffe9e6570e6ffa7dda68ab26be7aed86afd
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>