mptcp: move address attribute into mptcp_addr_info

So that can be accessed easily from the subflow creation
helper. No functional change intended.

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>