x86: disable X86_PTRACE_BTS for now

Oleg Nesterov found a couple of races in the ptrace-bts code
and fixes are queued up for it but they did not get ready in time
for the merge window. We'll merge them in v2.6.31 - until then
mark the feature as CONFIG_BROKEN. There's no user-space yet
making use of this so it's not a big issue.

Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>