xen/manage: don't complain about an empty value in control/sysrq node

When guest receives a sysrq request from the host it acknowledges it by
writing '\0' to control/sysrq xenstore node. This, however, make xenstore
watch fire again but xenbus_scanf() fails to parse empty value with "%c"
format string:

sysrq: SysRq : Emergency Sync
Emergency Sync complete
xen:manage: Error -34 reading sysrq code in control/sysrq

Ignore -ERANGE the same way we already ignore -ENOENT, empty value in
control/sysrq is totally legal.

Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Reviewed-by: Wei Liu <[e-mail]>
Signed-off-by: Boris Ostrovsky <[e-mail]>

drivers/xen/manage.c

···
 		/*
 		 * The Xenstore watch fires directly after registering it and
 		 * after a suspend/resume cycle. So ENOENT is no error but
-		 * might happen in those cases.
+		 * might happen in those cases. ERANGE is observed when we get
+		 * an empty value (''), this happens when we acknowledge the
+		 * request by writing '\0' below.
 		 */
-		if (err != -ENOENT)
+		if (err != -ENOENT && err != -ERANGE)
 			pr_err("Error %d reading sysrq code in control/sysrq\n",
 			       err);
 		xenbus_transaction_end(xbt, 1);