x86, perf_counter, bts: Fail if BTS is not available

Reserve PERF_COUNT_HW_BRANCH_INSTRUCTIONS with sample_period ==
1 for BTS tracing and fail, if BTS is not available.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090902140612.943801000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>