mptcp: netlink: fix some error return code

Fix to return negative error code -EINVAL from some error handling
case instead of 0, as done elsewhere in those functions.

Fixes: 9ab4807c ("mptcp: netlink: Add MPTCP_PM_CMD_ANNOUNCE")
Fixes: 702c2f64 ("mptcp: netlink: allow userspace-driven subflow establishment")
Cc: stable@vger.kernel.org
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>