With comments, clarify the code paths where a CM Service use count has
not yet been placed on the conn (just send CM Service Reject) and where
the use count is placed (decrement count on CM Service Reject).
Place the CM Service use count slightly earlier:
- it is then correctly present when checking the mobile identity in
cm_serv_reuse_conn(), avoiding the crash reported in OS#5532.
- there is only one place incrementing the use count instead of two.
Related: OS#5532
Change-Id: I6c735b79b67108bcaadada3f01c7046e262f939b