tracing: Add stack_tracer_disable/enable() functions

There are certain parts of the kernel that cannot let stack tracing
proceed (namely in RCU), because the stack tracer uses RCU, and parts of RCU
internals cannot handle having RCU read side locks taken.

Add stack_tracer_disable() and stack_tracer_enable() functions to let RCU
stop stack tracing on the current CPU when it is in those critical sections.

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