ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs

ipv6: convert CONFIG_IPV6 to built-in only and clean up Kconfigs

Maintaining a modular IPv6 stack offers image size savings for specific
setups, this benefit is outweighed by the architectural burden it
imposes on the subsystems on implementation and maintenance. Therefore,
drop it.

Change CONFIG_IPV6 from tristate to bool. Remove all Kconfig
dependencies across the tree that explicitly checked for IPV6=m. In
addition, remove MODULE_DESCRIPTION(), MODULE_ALIAS(), MODULE_AUTHOR()
and MODULE_LICENSE().

This is also replacing module_init() by device_initcall(). It is not
possible to use fs_initcall() as IPv4 does because that creates a race
condition on IPv6 addrconf.

Finally, modify the default configs from CONFIG_IPV6=m to CONFIG_IPV6=y
except for m68k as according to the bloat-o-meter the image is
increasing by 330KB~ and that isn't acceptable. Instead, disable IPv6 on
this architecture by default. This is aligned with m68k RAM requirements
and recommendations [1].

[1] http://www.linux-m68k.org/faq/ram.html

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org> # arm64
Link: https://patch.msgid.link/20260325120928.15848-2-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>