x86_64: Fix compat emulation of PTRACE_GET/SET_THREAD_AREA

Since the 64bit kernel has different indexes for this TLS segments
the address needs to be adjusted in the ptrace 32bit emulation.

[ tglx: arch/x86 adaptation ]

Reported-by: Amnon Shiloh
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>