powerpc/rtas: Serialize firmware activation sequences

Use rtas_ibm_activate_firmware_lock to prevent interleaving call
sequences of the ibm,activate-firmware RTAS function, which typically
requires multiple calls to complete the update. While the spec does
not specifically prohibit interleaved sequences, there's almost
certainly no advantage to allowing them.

Reviewed-by: "Aneesh Kumar K.V (IBM)" <[e-mail]>
Signed-off-by: Nathan Lynch <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/kernel/rtas.c

···
 		return;
 	}
 
+	mutex_lock(&rtas_ibm_activate_firmware_lock);
+
 	do {
 		fwrc = rtas_call(token, 0, 1, NULL);
 	} while (rtas_busy_delay(fwrc));
+
+	mutex_unlock(&rtas_ibm_activate_firmware_lock);
 
 	if (fwrc)
 		pr_err("ibm,activate-firmware failed (%i)\n", fwrc);