x86/ftrace: Make function graph use ftrace directly

x86/ftrace: Make function graph use ftrace directly

We don't need special hook for graph tracer entry point,
but instead we can use graph_ops::func function to install
the return_hooker.

This moves the graph tracing setup _before_ the direct
trampoline prepares the stack, so the return_hooker will
be called when the direct trampoline is finished.

This simplifies the code, because we don't need to take into
account the direct trampoline setup when preparing the graph
tracer hooker and we can allow function graph tracer on entries
registered with direct trampoline.

Link: https://lkml.kernel.org/r/[email protected]
[fixed compile error reported by kernel test robot <[email protected]>]
Signed-off-by: Steven Rostedt (VMware) <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>