net: Add sk_clone().

net: Add sk_clone().

sctp_accept() will use sk_clone_lock(), but it will be called
with the parent socket locked, and sctp_migrate() acquires the
child lock later.

Let's add no lock version of sk_clone_lock().

Note that lockdep complains if we simply use bh_lock_sock_nested().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20251023231751.4168390-5-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>