x86/kgbd: Use NMI_VECTOR not APIC_DM_NMI

apic->send_IPI_allbutself() takes a vector number as argument.

APIC_DM_NMI is clearly not a vector number. It's defined to 0x400 which is
outside the vector space.

Use NMI_VECTOR instead as that's what it is intended to be.

Fixes: 82da3ff8 ("x86: kgdb support")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190722105218.855189979@linutronix.de

arch/x86/kernel/kgdb.c

@@ -416,7 +416,7 @@ static void kgdb_disable_hw_debug(struct pt_regs *regs)
  */
 void kgdb_roundup_cpus(void)
 {
-	apic->send_IPI_allbutself(APIC_DM_NMI);
+	apic->send_IPI_allbutself(NMI_VECTOR);
 }
 #endif