KVM: arm64: nv: Unfudge ID_AA64PFR0_EL1 masking

Marc reports that L1 VMs aren't booting with the NV series applied to
today's kvmarm/next. After bisecting the issue, it appears that
44241f34 ("KVM: arm64: nv: Use accessors for modifying ID
registers") is to blame.

Poking around at the issue a bit further, it'd appear that the value for
ID_AA64PFR0_EL1 is complete garbage, as 'val' still contains the value
we set ID_AA64ISAR1_EL1 to.

Fix the read-modify-write pattern to actually use ID_AA64PFR0_EL1 as the
starting point. Excuse me as I return to my shame cube.
Reported-by: Marc Zyngier <maz@kernel.org>
Fixes: 44241f34 ("KVM: arm64: nv: Use accessors for modifying ID registers")
Acked-by: Marc Zyngier <maz@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240621224044.2465901-1-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/nested.c

@@ -40,7 +40,7 @@ static void limit_nv_id_regs(struct kvm *kvm)
 	kvm_set_vm_id_reg(kvm, SYS_ID_AA64ISAR1_EL1, val);
 
 	/* No AMU, MPAM, S-EL2, RAS or SVE */
-	kvm_read_vm_id_reg(kvm, SYS_ID_AA64PFR0_EL1);
+	val = kvm_read_vm_id_reg(kvm, SYS_ID_AA64PFR0_EL1);
 	val &= ~(GENMASK_ULL(55, 52)	|
 		 NV_FTR(PFR0, AMU)	|
 		 NV_FTR(PFR0, MPAM)	|