x86, ftrace, hw-branch-tracer: reset trace buffer on close

Reset the ftrace buffer on close. Since we use cyclic buffers, the
trace is not contiguous, anyway.

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>