mptcp: sockopt: make 'tcp_fastopen_connect' generic

mptcp: sockopt: make 'tcp_fastopen_connect' generic

[ Upstream commit d3d429047cc66ff49780c93e4fccd9527723d385 ]

There are other socket options that need to act only on the first
subflow, e.g. all TCP_FASTOPEN* socket options.

This is similar to the getsockopt version.

In the next commit, this new mptcp_setsockopt_first_sf_only() helper is
used by other another option.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: 21e43569685d ("mptcp: fix locking for setsockopt corner-case")
Signed-off-by: Sasha Levin <sashal@kernel.org>