[PATCH] x86_64: Fix canonical checking for segment registers in ptrace

Allowed user programs to set a non canonical segment base, which would cause
oopses in the kernel later.

Credit-to: Alexander Nyberg <alexn@dsv.su.se>

 For identifying and reporting this bug.

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