tcp: only release congestion control if it has been initialized

tcp: only release congestion control if it has been initialized

Currently, when cleaning up congestion control, we always call the
release regardless of whether it has been initialized. There is no
need to release when closing TCP_LISTEN and TCP_CLOSE (close
immediately after socket()).

In this case, tcp_cdg calls kfree(NULL) in release without causing
an exception, but for some customized ca, this could lead to
unexpected exceptions. We need to ensure that init and release are
called in pairs.

Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Link: https://patch.msgid.link/1729845944-6003-1-git-send-email-yangpc@wangsu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>