net/ipv6/addrconf: IPv6 tethering enhancement

Added new procfs flag to toggle the automatic addition of prefix
routes on a per device basis. The new flag is accept_ra_prefix_route.
Defaults to 1 as to not break existing behavior.

Change-Id: If25493890c7531c27f5b2c4855afebbbbf5d072a
CRs-Fixed: 504095
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>