KVM: arm64: selftest: Perform ISB before reading PAR_EL1

It looks like a mistake to issue ISB *after* reading PAR_EL1, we should
instead perform it between the AT instruction and the reads of PAR_EL1.

As according to DDI0487J.a IJTYVP,

"When an address translation instruction is executed, explicit
 synchronization is required to guarantee the result is visible to
 subsequent direct reads of PAR_EL1."

Otherwise all guest_at testcases fail on my box with

==== Test Assertion Failure ====
  aarch64/page_fault_test.c:142: par & 1 == 0
  pid=1355864 tid=1355864 errno=4 - Interrupted system call
     1	0x0000000000402853: vcpu_run_loop at page_fault_test.c:681
     2	0x0000000000402cdb: run_test at page_fault_test.c:730
     3	0x0000000000403897: for_each_guest_mode at guest_modes.c:100
     4	0x00000000004019f3: for_each_test_and_guest_mode at page_fault_test.c:1105
     5	 (inlined by) main at page_fault_test.c:1131
     6	0x0000ffffb153c03b: ?? ??:0
     7	0x0000ffffb153c113: ?? ??:0
     8	0x0000000000401aaf: _start at ??:?
  0x1 != 0x0 (par & 1 != 0)

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231007124043.626-2-yuzenghui@huawei.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

tools/testing/selftests/kvm/aarch64/page_fault_test.c

@@ -135,8 +135,8 @@ static void guest_at(void)
 	uint64_t par;
 
 	asm volatile("at s1e1r, %0" :: "r" (guest_test_memory));
-	par = read_sysreg(par_el1);
 	isb();
+	par = read_sysreg(par_el1);
 
 	/* Bit 1 indicates whether the AT was successful */
 	GUEST_ASSERT_EQ(par & 1, 0);