diag: Fix race condition in diagchar driver

diag driver maintains socket logging process in global driver
structure which is manipulated in switch_logging and in close
file_operation.
There is a possibility of race while clearing the socket logging
process in close operation and sending signal in switch_logging
operation. Adding lock to serialize the two operations.

CRs-Fixed: 495063
Change-Id: I0064509c9ace18a8ff40c5a744363546a9021612
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>