selftests: mptcp: set and print the fullmesh flag

This patch dealt with the MPTCP_PM_ADDR_FLAG_FULLMESH flag in add_addr()
and print_addr(), to set and print out the fullmesh flag.

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