diag: Fix possible access to freed variable

Mdlog structure member - hdlc timer is being accessed after
mdlog session is terminated.
This patch prevents the access to freed variable
by providing session_lock protection.

Change-Id: I1701e57fb4bbb751b76a4f24dc936ac22524e26c
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>