diag: dci: Mutex protect the client details

While updating the global mask details, the client entry is not
mutex protected. It may happen in some cases that stale entries
might be accessed. This patch ensures that updation of global
mask details happens under mutex protection.

Change-Id: I614872c2bd73ffd53368490a52a54dad13b08e70
Signed-off-by: Katish Paran <kparan@codeaurora.org>