x86, perf_counter, bts: Do not allow kernel BTS tracing for now

Kernel BTS tracing generates too much data too fast for us to
handle, causing the kernel to hang.

Fail for BTS requests for kernel code.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Acked-by: Peter Zijlstra <a.p.zjilstra@chello.nl>
LKML-Reference: <20090902140616.901253000@intel.com>
[ This is really a workaround - but we want BTS tracing in .32
  so make sure we dont regress. The lockup should be fixed
  ASAP. ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>