arm64: cpufeature: correctly handle MRS to XZR

In emulate_mrs() we may erroneously write back to the user SP rather
than XZR if we trap an MRS instruction where Xt == 31.

Use the new pt_regs_write_reg() helper to handle this correctly.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 77c97b4e ("arm64: cpufeature: Expose CPUID registers by emulation")
Cc: Andre Przywara <andre.przywara@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/cpufeature.c

@@ -1214,7 +1214,7 @@ static int emulate_mrs(struct pt_regs *regs, u32 insn)
 	rc = emulate_sys_reg(sys_reg, &val);
 	if (!rc) {
 		dst = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RT, insn);
-		regs->user_regs.regs[dst] = val;
+		pt_regs_write_reg(regs, dst, val);
 		regs->pc += 4;
 	}