ftrace: prevent ftrace_special from recursion

Impact: stop ftrace_special from recursion

The ftrace_special is used to help debug areas of the kernel.
Because of this, if it is put in certain locations, the fact that
it allows recursion can become a problem if the kernel developer
using does not realize that.

This patch changes ftrace_special to not allow recursion into itself
to make it more robust.

It also changes from preempt disable interrupts disable to prevent
any loss of trace entries.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>