tracepoint: Use __idx instead of idx in DO_TRACE macro to make it unique

After enabling KVM event tracing, almost all of trace_kvm_exit()'s
printk shows

	"kvm_exit: IRQ: ..."

even if the actual exception_type is NOT IRQ.  More specifically,
trace_kvm_exit() is defined in virt/kvm/arm/trace.h by TRACE_EVENT.

This slight problem may have existed after commit e6753f23
("tracepoint: Make rcuidle tracepoint callers use SRCU"). There are
two variables in trace_kvm_exit() and __DO_TRACE() which have the
same name, *idx*. Thus the actual value of *idx* will be overwritten
when tracing. Fix it by adding a simple prefix.

Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Wang Haibin <wanghaibin.wang@huawei.com>
Cc: linux-trace-devel@vger.kernel.org
Cc: stable@vger.kernel.org
Fixes: e6753f23

 ("tracepoint: Make rcuidle tracepoint callers use SRCU")
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>