ipv6: Enable new mode proxy_ndp == 2

This new mode allows Neighbor discovery packets to be sent to userspace
without regards to the state of the forwarding setting. Without this, NDP
packets addressed to the host are still received, but those for other
addresses are not.

Enabling this mode allows NDP proxying to be performed from userspace.

Change-Id: I69b7a7c0c42e3253c42d6f2e163c0ce1d848aed6
Signed-off-by: Jaime Lopez <jaimel@codeaurora.org>