mptcp: change the mpc check helper to return a sk

After the previous patch the __mptcp_nmpc_socket helper is used
only to ensure that the MPTCP socket is a suitable status - that
is, the mptcp capable handshake is not started yet.

Change the return value to the relevant subflow sock, to finally
remove the last references to first subflow socket in the MPTCP stack.

As a bonus, we can get rid of a few local variables in different
functions.

No functional change 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>