ftrace: Access ret_stack->subtime only in the function profiler

ftrace: Access ret_stack->subtime only in the function profiler

The subtime is used only for function profiler with function graph
tracer enabled.  Move the definition of subtime under
CONFIG_FUNCTION_PROFILER to reduce the memory usage.  Also move the
initialization of subtime into the graph entry callback.

Link: http://lkml.kernel.org/r/[email protected]
Cc: Ingo Molnar <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>