tracing: Add tp_printk cmdline to have tracepoints go to printk()

Add the kernel command line tp_printk option that will have tracepoints
that are active sent to printk() as well as to the trace buffer.

Passing "tp_printk" will activate this. To turn it off, the sysctl
/proc/sys/kernel/tracepoint_printk can have '0' echoed into it. Note,
this only works if the cmdline option is used. Echoing 1 into the sysctl
file without the cmdline option will have no affect.

Note, this is a dangerous option. Having high frequency tracepoints send
their data to printk() can possibly cause a live lock. This is another
reason why this is only active if the command line option is used.

Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1412121539300.16494@nanos

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>