Impact: fix crash when enabling the branch-tracer

When the branch tracer inserts an event through
probe_likely_condition(), it calls local_irq_save() and then results
in a trace recursion.

local_irq_save() -> trace_hardirqs_off() -> trace_hardirqs_off_caller()
	-> unlikely()

The trace_branch.c file is protected by DISABLE_BRANCH_PROFILING but
that doesn't prevent from external call to functions that use
unlikely().

My box crashed each time I tried to set this tracer (sudden and hard
reboot).
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>