tracing: Initialize upper and lower vars in pid_list_refill_irq()

The upper and lower variables are set as link lists to add into the sparse
array. If they are NULL, after the needed allocations are done, then there
is nothing to add. But they need to be initialized to NULL for this to
work.

Link: https://lore.kernel.org/all/221bc7ba-a475-1cb9-1bbe-730bb9c2d448@canonical.com/

Fixes: 8d6e909 ("tracing: Create a sparse bitmask for pid filtering")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/pid_list.c

@@ -333,8 +333,8 @@ static void pid_list_refill_irq(struct irq_work *iwork)
 {
 	struct trace_pid_list *pid_list = container_of(iwork, struct trace_pid_list,
 						       refill_irqwork);
-	union upper_chunk *upper;
-	union lower_chunk *lower;
+	union upper_chunk *upper = NULL;
+	union lower_chunk *lower = NULL;
 	union upper_chunk **upper_next = &upper;
 	union lower_chunk **lower_next = &lower;
 	int upper_count;