dm verity: set DM_TARGET_IMMUTABLE feature flag

dm verity: set DM_TARGET_IMMUTABLE feature flag

The device-mapper framework provides a mechanism to mark targets as
immutable (and hence fail table reloads that try to change the target
type). Add the DM_TARGET_IMMUTABLE flag to the dm-verity target's
feature flags to prevent switching the verity target with a different
target type.

Fixes: a4ffc152198e ("dm: add verity target")
Cc: [email protected]
Signed-off-by: Sarthak Kukreti <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>