[PATCH] x86_64: Don't allow accesses below register frame in ptrace

There was a "off by one quad word" error in there.  I don't think it is
exploitable because it will only store into a unused area, but better to plug
it.

Found and fixed by John Blackwood

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>