mm: export use_mm/unuse_mm to modules

vhost net module wants to do copy to/from user from a kernel thread,
which needs use_mm. Export it to modules.

Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>