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