ANDROID: dm: android-verity: rebase for 4.9

Export the direct_access method of dm_linear target for
dm-android-verity target.

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: I46556d882305e5194352946264cbc9c06e5038e4

[AmitP: Rebased the changes for v4.14]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>