x86_64: Don't leak NT bit into next task (CVE-2006-5755)

x86_64: Don't leak NT bit into next task (CVE-2006-5755)

SYSENTER can cause a NT to be set which might cause crashes on the IRET
in the next task.

Following similar i386 patch from Linus.

Backport to 2.6.16 by Chuck Ebbert <76306.1226@compuserve.com>
[Changed 'set_debugreg' to the older 'set_debug' in setup64.c
and added raw_local_save_flags() from 2.6.19 to system.h]

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>