perf/x86/intel/pt: Allow the disabling of branch tracing

perf/x86/intel/pt: Allow the disabling of branch tracing

Now that Intel PT supports more types of trace content than just branch
tracing, it may be useful to allow the user to disable branch tracing
when it is not needed.

The special case is BDW, where not setting BranchEn is not supported.

This is slightly trickier than necessary, because up to this moment
the driver has been setting BranchEn automatically and the userspace
assumes as much. Instead of reversing the semantics of BranchEn, we
introduce a 'passthrough' bit, which will forego the default and allow
the user to set BranchEn to their heart's content.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: vince@deater.net
Link: http://lkml.kernel.org/r/20170206144140.14402-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>