x86: disable X86_PTRACE_BTS

there's a new ptrace arch level feature in .28:

  config X86_PTRACE_BTS
  bool "Branch Trace Store"

it has broken fork() handling: the old DS area gets copied over into
a new task without clearing it.

Fixes exist but they came too late:

  c5dee617: x86, bts: memory accounting
  bf53de90: x86, bts: add fork and exit handling

and are queued up for v2.6.29. This shows that the facility is still not
tested well enough to release into a stable kernel - disable it for now and
reactivate in .29. In .29 the hardware-branch-tracer will use the DS/BTS
facilities too - hopefully resulting in better code.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/Kconfig.cpu

@@ -520,6 +520,7 @@ config X86_PTRACE_BTS
 	bool "Branch Trace Store"
 	default y
 	depends on X86_DEBUGCTLMSR
+	depends on BROKEN
 	help
 	  This adds a ptrace interface to the hardware's branch trace store.