mptcp: drop OPTION_MPTCP_ADD_ADDR6

Since the family field was added in struct mptcp_out_options, no need to
use OPTION_MPTCP_ADD_ADDR6 to identify the IPv6 address. Drop it.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>