DO NOT MERGE: Revert "Don't start tethering if IPv6 RPF is not supported."

This breaks tethering on kernels that do not have the necessary
configuration option enabled. This change can be reverted once
the AOSP kernels have been updated.

This reverts commit 219672ac.

Test: Manually started hotspot on bullhead with N-vintage kernel.
Change-Id: Ib1397054b309f6ed023930c82ff15d92f088590d