net: introduce EXPORT_IPV6_MOD() and EXPORT_IPV6_MOD_GPL()

net: introduce EXPORT_IPV6_MOD() and EXPORT_IPV6_MOD_GPL()

We have many EXPORT_SYMBOL(x) in networking tree because IPv6
can be built as a module.

CONFIG_IPV6=y is becoming the norm.

Define a EXPORT_IPV6_MOD(x) which only exports x
for modular IPv6.

Same principle applies to EXPORT_IPV6_MOD_GPL()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Link: https://patch.msgid.link/20250212132418.1524422-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>