ANDROID: dm: rename dm-linear methods for dm-android-verity

This keeps linear_target as static variable and just exposes
the linear target methods for android-verity

Cherry-picked: https://android-review.googlesource.com/#/c/212858



Change-Id: I0de4ad485ce3fd21402be2ca3af39fa0d11ccd17
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Git-commit: fd0e6dcf
Git-repo: https://git.linaro.org/kernel/linux-linaro-stable.git


Signed-off-by: Imran Khan <kimran@codeaurora.org>