diag: Prevent possible deadlock while memory device session runs

Possibility of deadlock trying to acquire a mutex when memory
device session is active is prevented by proper synchronization of
mutexes involved in the threads.

Change-Id: I4dca3096f37bf2208f1e0cc7baa59ba288bb1496
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>