tracing: Do note expose stack_trace_filter without DYNAMIC_FTRACE

The "stack_trace_filter" file only makes sense if DYNAMIC_FTRACE is
configured in. If it is not, then the user can not filter any functions.

Not only that, the open function causes warnings when DYNAMIC_FTRACE is not
set.

Link: http://lkml.kernel.org/r/20170710110521.600806-1-arnd@arndb.de



Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>