diag: Use file's private data to get pid of exiting process

Close to diag fd can be called from a work function having
different pid from process who has registered with diag.
This is resulting in not removing cmd registration for exiting
process. The patch fixes this issue with using file's private
data to get pid of exiting process.

Change-Id: I1036b9e159b1668558da62fe7588acf1367d6aeb
Signed-off-by: Hardik Arya <harya@codeaurora.org>
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>