mptcp: allow creating non-backup subflows

Currently the 'backup' attribute of local endpoint
is ignored. Let's use it for the MP_JOIN handshake

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>