net: qrtr: get svc_id before queueing sk_buff

In qrtr_endpoint_post, getting svc_id based on sk_buff->cb after sk_buff
has been queued to the endpoint leads to a potential use-after-free
scenario. To avoid this race condition, get the service ID via
sk_buff->cb before queueing the sk_buff.

Change-Id: I53205fdffc08fd6dc48fd158c7fe5966f38aa978
Signed-off-by: Tony Truong <quic_truong@quicinc.com>