mptcp: add fullmesh check for adding address

mptcp: add fullmesh check for adding address

The fullmesh flag mustn't be used with the signal flag when adding an
address. Commands like this should be treated as invalid commands:

 ip mptcp endpoint add 10.0.2.1 signal fullmesh

This patch added the necessary flags check for this case.

Acked-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: David Ahern <dsahern@kernel.org>