x86/debug: Handle early WARN_ONs proper

Hans managed to trigger a WARN very early in the boot which killed his
(Virtual) box.

The reason is that the recent rework of WARN() to use UD0 forgot to add the
fixup_bug() call to early_fixup_exception(). As a result the kernel does
not handle the WARN_ON injected UD0 exception and panics.

Add the missing fixup call, so early UD's injected by WARN() get handled.

Fixes: 9a93848f ("x86/debug: Implement __WARN() using UD0")
Reported-and-tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Frank Mehnert <frank.mehnert@oracle.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Michael Thayer <michael.thayer@oracle.com>
Link: http://lkml.kernel.org/r/20170612180108.w4vgu2ckucmllf3a@hirez.programming.kicks-ass.net