ANDROID: android-verity: mark dev as rw for linear target

Mark as rw when adding as linear target to allow changes
to the underlying filesystem through adb disable verity
and adb remount.

(Cherry-picked from
https://partner-android-review.googlesource.com/#/c/613573/


79a3032bb62da65a5d724eb70c8bdc662945d475)

BUG: 28845874
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: If41e9cad8e0f054f4778c09a6e2f0cb8af6fddaf
Git-commit: 492a6047
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Runmin Wang <runminw@codeaurora.org>