markers: fix sparse integer as NULL pointer warning

kernel/trace/trace_sysprof.c:164:20: warning: Using plain integer as NULL pointer
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/trace/trace_sysprof.c

@@ -161,7 +161,7 @@ static void timer_notify(struct pt_regs *regs, int cpu)
 		__trace_special(tr, data, 2, regs->ip, 0);
 
 		while (i < sample_max_depth) {
-			frame.next_fp = 0;
+			frame.next_fp = NULL;
 			frame.return_address = 0;
 			if (!copy_stack_frame(fp, &frame))
 				break;