mptcp: avoid additional indirection in sockopt

The mptcp sockopt infrastructure unneedly uses the first subflow
socket struct in a few spots. We are going to remove such field
soon, so use directly the first subflow sock instead.

No functional changes intended.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>