ftrace: Only update the function code on write to filter files

commit 058e297d34a404caaa5ed277de15698d8dc43000 upstream.

If function tracing is enabled, a read of the filter files will
cause the call to stop_machine to update the function trace sites.
It should only call stop_machine on write.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>