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>