tracing: Introduce TRACE_EVENT_FN_COND macro

TRACE_EVENT_FN can't be used in some circumstances
like invoking trace functions from offlined CPU due
to RCU usage.

This patch adds the TRACE_EVENT_FN_COND macro
to make such trace points conditional.

Link: http://lkml.kernel.org/r/1450124286-4822-1-git-send-email-kda@linux-powerpc.org



Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>