mptcp: introduce 'sk' to replace 'sock->sk' in mptcp_listen()

[ Upstream commit cfdcfeed ]

'sock->sk' is used frequently in mptcp_listen(). Therefore, we can
introduce the 'sk' and replace 'sock->sk' with it.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 0226436a ("mptcp: do not rely on implicit state check in mptcp_listen()")
Signed-off-by: Sasha Levin <sashal@kernel.org>