mptcp: add fullmesh flag check for adding address

The fullmesh flag mustn't be used with the signal flag when adding an
address. This patch added the necessary flags check for this case.

Fixes: 73c762c1 ("mptcp: set fullmesh flag in pm_netlink")
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>