x86/traps: die() instead of panicking on a double fault

x86/traps: die() instead of panicking on a double fault

A double fault has a decent chance of being recoverable by killing
the offending thread.  Use die() so that we at least try to recover.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>