ANDROID: ashmem: fix up vm_flags direct access which is not allowed anymore

Commit 1c71222e ("mm: replace vma->vm_flags direct modifications
with modifier calls") removed the ability to directly access vm_flags, but
provided wrapper functions to do the task instead.  Use vm_flags_clear()
to fix the build breakage in the ashmem code because of this change.

Fixes: 1c71222e ("mm: replace vma->vm_flags direct modifications with modifier calls")
Cc: Suren Baghdasaryan <surenb@google.com>
Change-Id: I865469c1eacd8d037eaaa85eca65e54f717e8706
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>