ftrace/graph: Add ftrace_graph_max_depth kernel parameter

ftrace/graph: Add ftrace_graph_max_depth kernel parameter

Early trace callgraphs can be extremely large on systems with
several seconds of boot time. The max_depth parameter limits how
deep the graph trace goes and reduces the output size. This
parameter is the same as the max_graph_depth file in tracefs.

Link: http://lkml.kernel.org/r/1488499935-23216-1-git-send-email-todd.e.brandt@linux.intel.com
Signed-off-by: Todd Brandt <todd.e.brandt@linux.intel.com>
[ changed comments about debugfs to tracefs ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>