ptrace: flush breakpoints

Unfortunately the kernel doesn't flush hw breakpoints on
detaching ptrace. If a breakpoint is triggered without ptrace, it
will be killed by SIGTRAP.

Reported-by: Mr Jenkins
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>