tracing: Remove outdated comment in stack handling

This comment describes the behaviour before commit 2a820bf749
("tracing: Use percpu stack trace buffer more intelligently").  Since
that commit, interrupts and NMIs do use the per-cpu stacks so the
comment is no longer correct.  Remove it.

(Note that the FTRACE_STACK_SIZE mentioned in the comment has never
existed, it probably should have said FTRACE_STACK_ENTRIES.)

Link: https://lkml.kernel.org/r/20200727092840.18659-1-vincent.whitchurch@axis.com

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2930,12 +2930,6 @@ static void __ftrace_trace_stack(struct trace_buffer *buffer,
 		skip++;
 #endif
 
-	/*
-	 * Since events can happen in NMIs there's no safe way to
-	 * use the per cpu ftrace_stacks. We reserve it and if an interrupt
-	 * or NMI comes in, it will just have to use the default
-	 * FTRACE_STACK_SIZE.
-	 */
 	preempt_disable_notrace();
 
 	stackidx = __this_cpu_inc_return(ftrace_stack_reserve) - 1;