ipv6: Simplify link-local address generation for IPv6 GRE.

ipv6: Simplify link-local address generation for IPv6 GRE.

Since commit 3e6a0243ff00 ("gre: Fix again IPv6 link-local address
generation."), addrconf_gre_config() has stopped handling IP6GRE
devices specially and just calls the regular addrconf_addr_gen()
function to create their link-local IPv6 addresses.

We can thus avoid using addrconf_gre_config() for IP6GRE devices and
use the normal IPv6 initialisation path instead (that is, jump directly
to addrconf_dev_config() in addrconf_init_auto_addrs()).

See commit 3e6a0243ff00 ("gre: Fix again IPv6 link-local address
generation.") for a deeper explanation on how and why GRE devices
started handling their IPv6 link-local address generation specially,
why it was a problem, and why this is not even necessary in most cases
(especially for GRE over IPv6).

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/a9144be9c7ec3cf09f25becae5e8fdf141fde9f6.1750075076.git.gnault@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>