[PATCH] x86_64: Clean up double fault handling

Since a double fault always implies that kernel data structures are
corrupt, this fault should neither be handed to user mode handling,
nor should the handler allow resuming the faulting code stream (since
architecturally this isn't a fault, but an abort).

Note that this slightly depends on the previously submitted patch
adjusting the prototype of notify_die() (a compiler warning will result
without that other patch).

AK: Removed obsolete CONFIG_CHECKING code, added comments

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