powerpc/rtas: enture rtas_call is called with MMU enabled

rtas_call must not be called with the MMU disabled because in case
of rtas error, log_error is called which requires MMU enabled. Add
a test and warning for this.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220308135047.478297-14-npiggin@gmail.com

arch/powerpc/kernel/rtas.c

@@ -476,6 +476,11 @@ int rtas_call(int token, int nargs, int nret, int *outputs, ...)
 	if (!rtas.entry || token == RTAS_UNKNOWN_SERVICE)
 		return -1;
 
+	if ((mfmsr() & (MSR_IR|MSR_DR)) != (MSR_IR|MSR_DR)) {
+		WARN_ON_ONCE(1);
+		return -1;
+	}
+
 	s = lock_rtas();
 
 	/* We use the global rtas args buffer */