KVM: arm/arm64: vgic-v3: Fix nr_pre_bits bitfield extraction

We used to extract PRIbits from the ICH_VT_EL2 which was the upper field
in the register word, so a mask wasn't necessary, but as we switched to
looking at PREbits, which is bits 26 through 28 with the PRIbits field
being potentially non-zero, we really need to mask off the field value,
otherwise fun things may happen.
Signed-off-by: Christoffer Dall <cdall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>

virt/kvm/arm/hyp/vgic-v3-sr.c

@@ -22,7 +22,7 @@
 #include <asm/kvm_hyp.h>
 
 #define vtr_to_max_lr_idx(v)		((v) & 0xf)
-#define vtr_to_nr_pre_bits(v)		(((u32)(v) >> 26) + 1)
+#define vtr_to_nr_pre_bits(v)		((((u32)(v) >> 26) & 7) + 1)
 
 static u64 __hyp_text __gic_v3_get_lr(unsigned int lr)
 {