smcinvoke : Add locking to shared variables

This change adds locking to shared variables to
prevent any data race conditions. cb_reqs_inflight
and server_info should both be protected under locks.

Change-Id: I3475ab3208c2fc91fce2865d64626efdcd404e70
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>