perf: Fix irq_work 'tail' recursion

Vince reported a watchdog lockup like:

	[<ffffffff8115e114>] perf_tp_event+0xc4/0x210
	[<ffffffff810b4f8a>] perf_trace_lock+0x12a/0x160
	[<ffffffff810b7f10>] lock_release+0x130/0x260
	[<ffffffff816c7474>] _raw_spin_unlock_irqrestore+0x24/0x40
	[<ffffffff8107bb4d>] do_send_sig_info+0x5d/0x80
	[<ffffffff811f69df>] send_sigio_to_task+0x12f/0x1a0
	[<ffffffff811f71ce>] send_sigio+0xae/0x100
	[<ffffffff811f72b7>] kill_fasync+0x97/0xf0
	[<ffffffff8115d0b4>] perf_event_wakeup+0xd4/0xf0
	[<ffffffff8115d103>] perf_pending_event+0x33/0x60
	[<ffffffff8114e3fc>] irq_work_run_list+0x4c/0x80
	[<ffffffff8114e448>] irq_work_run+0x18/0x40
	[<ffffffff810196af>] smp_trace_irq_work_interrupt+0x3f/0xc0
	[<ffffffff816c99bd>] trace_irq_work_interrupt+0x6d/0x80

Which is caused by an irq_work generating new irq_work and therefore
not allowing forward progress.

This happens because processing the perf irq_work triggers another
perf event (tracepoint stuff) which in turn generates an irq_work ad
infinitum.

Avoid this by raising the recursion counter in the irq_work -- which
effectively disables all software events (including tracepoints) from
actually triggering again.

Reported-by: Vince Weaver <vincent.weaver@maine.edu>
Tested-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20150219170311.GH21418@twins.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>