dccp: Inline dccp_listen_start().

This patch inlines dccp_listen_start() and removes a stale comment in
inet_dccp_listen() so that it looks like inet_listen().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Reviewed-by: Richard Sailer <richard_siegfried@systemli.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>