diag: Fix race condition while accessing mdlog session info

The patch avoids possible race condition while
accessing mdlog session info by synchronizing across
different mdlog session info access.

CRs-Fixed: 2022664
Change-Id: Ia10e83c7376de5cab92dd2ec4ec4aa8074223ff3
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>