net: Unexport shared functions for DCCP.

DCCP was removed, so many inet functions no longer need to
be exported.

Let's unexport or use EXPORT_IPV6_MOD() for such functions.

sk_free_unlock_clone() is inlined in sk_clone_lock() as it's
the only caller.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250410023921.11307-4-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>