mptcp: allow to use port and non-signal in set_flags

It's illegal to use both port and non-signal flags for adding address.
But it's legal to use both of them for setting flags, which always uses
non-signal flags, backup or fullmesh.

This patch moves this non-signal flag with port check from
mptcp_pm_parse_addr() to mptcp_nl_cmd_add_addr(). Do the check only when
adding addresses, not setting flags or deleting addresses.

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>