x86/traps: Fix handling of #DB exceptions in hypervisor context

x86/traps: Fix handling of #DB exceptions in hypervisor context

The WARN_ON() can be triggered by guest activities, and emits a full stack
trace without rate limiting.  Swap it out for a ratelimited printk with just
enough information to work out what is going on.

Not all #DB exceptions are traps, so blindly continuing is not a safe action
to take.  We don't let PV guests select these settings in the real %dr7 to
begin with, but for added safety against unexpected situations, detect the
fault cases and crash in an obvious manner.

This is part of XSA-260 / CVE-2018-8897

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>