Merge branch 'do-not-allow-adding-routes-if-disable_ipv6-is-enabled'

Lorenzo Bianconi says:

====================
do not allow adding routes if disable_ipv6 is enabled

Do not allow userspace to add static ipv6 routes if disable_ipv6 is enabled.
Update disable_ipv6 documentation according to that change

Changes since v1:
- added an extack message telling the user that IPv6 is disabled on the nexthop
  device
- rebased on-top of net-next
====================

Signed-off-by: David S. Miller <davem@davemloft.net>