ip/ip6_gre: Fix changing addr gen mode not generating IPv6 link local address

[ Upstream commit 23ca0c2c ]

For our point-to-point GRE tunnels, they have IN6_ADDR_GEN_MODE_NONE
when they are created then we set IN6_ADDR_GEN_MODE_EUI64 when they
come up to generate the IPv6 link local address for the interface.
Recently we found that they were no longer generating IPv6 addresses.
This issue would also have affected SIT tunnels.

Commit e5dd7294 changed the code path so that GRE tunnels
generate an IPv6 address based on the tunnel source address.
It also changed the code path so GRE tunnels don't call addrconf_addr_gen
in addrconf_dev_config which is called by addrconf_sysctl_addr_gen_mode
when the IN6_ADDR_GEN_MODE is changed.

This patch aims to fix this issue by moving the code in addrconf_notify
which calls the addr gen for GRE and SIT into a separate function
and calling it in the places that expect the IPv6 address to be
generated.

The previous addrconf_dev_config is renamed to addrconf_eth_config
since it only expected eth type interfaces and follows the
addrconf_gre/sit_config format.

A part of this changes means that the loopback address will be
attempted to be configured when changing addr_gen_mode for lo.
This should not be a problem because the address should exist anyway
and if does already exist then no error is produced.

Fixes: e5dd7294

 ("ip/ip6_gre: use the same logic as SIT interfaces when computing v6LL address")
Signed-off-by: Thomas Winter <Thomas.Winter@alliedtelesis.co.nz>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>