diag: Prevent resource leakage while copying to userspace

Possibility of resource leakage while checking for validity of
pid and its task structures is prevented by proper update of pid
reference count.

Change-Id: Ifb38f91a5c3e45248bb08c5341c8a3095585c16f
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>