function-graph: move initialization of new tasks up in fork

When the function graph tracer is enabled, all new tasks must allocate
a ret_stack to place the return address of functions. This is because
the function graph tracer will replace the real return address with a
call to the tracing of the exit function.

This initialization happens in fork, but it happens too late. If fork
fails, then it will call free_task and that calls the freeing of this
ret_stack. But before initialization happens, the new (failed) task
points to its parents ret_stack. If a fork failure happens during
the function trace, it would be catastrophic for the parent.

Also, there's no need to call ftrace_graph_exit_task from fork, since
it is called by free_task which fork calls on failure.

[ Impact: prevent crash during failed fork running function graph tracer ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>