diag: Fix for possible stale task entries

The task entries were cleared while closing the
md session and hence are stale when notifying the client
with the task entries.
This patch provides the protection on md session and
also checks for the session pid and task pid being same.

CRs-Fixed: 1008137
Change-Id: I999db2865d10464c7f1ab4a5a940d23c725ac033
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>