powerpc/64s/kuap: Restore AMR in system reset exception

The system reset interrupt handler locks AMR and exits with
EXCEPTION_RESTORE_REGS without restoring AMR. Similarly to the
soft-NMI handler, it needs to restore.

Fixes: 890274c2 ("powerpc/64s: Implement KUAP for Radix MMU")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200429065654.1677541-5-npiggin@gmail.com

arch/powerpc/kernel/exceptions-64s.S

@@ -971,6 +971,7 @@ EXC_COMMON_BEGIN(system_reset_common)
 	ld	r10,SOFTE(r1)
 	stb	r10,PACAIRQSOFTMASK(r13)
 
+	kuap_restore_amr r10
 	EXCEPTION_RESTORE_REGS
 	RFI_TO_USER_OR_KERNEL