ftrace: change function graph tracer to use new in_nmi

The function graph tracer piggy backed onto the dynamic ftracer
to use the in_nmi custom code for dynamic tracing. The problem
was (as Andrew Morton pointed out) it really only wanted to bail
out if the context of the current CPU was in NMI context. But the
dynamic ftrace in_nmi custom code was true if _any_ CPU happened
to be in NMI context.

Now that we have a generic in_nmi interface, this patch changes
the function graph code to use it instead of the dynamic ftarce
custom code.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>