arm64: nmi: fix using wrong offset to get priority

hulk inclusion
category: bugfix
bugzilla: NA
CVE: NA

-------------------------------------------------

We used wrong offset to get the priority of sgi, then rewrite the sgi
prio to the wrong value by mistake.

Fixes: dec86aedbc8b("arm64: Add support for on-demand backtrace of other CPUs")
Signed-off-by: NWei Li <liwei391@huawei.com>
Reviewed-by: NYang Yingliang <yangyingliang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

arch/arm64/kernel/smp.c

@@ -1092,7 +1092,7 @@ void ipi_set_nmi_prio(void __iomem *base, u8 prio)
 
 	u32 offset = (IPI_CPU_BACKTRACE / 4) * 4;
 	u32 shift = (IPI_CPU_BACKTRACE % 4) * 8;
-	u32 prios = readl_relaxed(base + offset);
+	u32 prios = readl_relaxed(base + GICR_IPRIORITYR0 + offset);
 
 	/* clean old priority */
 	prios &= ~(0xff << shift);