ACPI / APEI: Fix the returned value in erst_dbg_read

If the persistent store is empty initially, the function 'erst_dbg_read'
returns a nonzero value. The better way is to return a zero indicating the
read operation reaches EOF.

Tested on two different servers.
Signed-off-by: Adrian Huang <adrian.huang@hp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/apei/erst-dbg.c

@@ -111,8 +111,17 @@ static ssize_t erst_dbg_read(struct file *filp, char __user *ubuf,
 	if (rc)
 		goto out;
 	/* no more record */
-	if (id == APEI_ERST_INVALID_RECORD_ID)
+	if (id == APEI_ERST_INVALID_RECORD_ID) {
+		/*
+		 * If the persistent store is empty initially, the function
+		 * 'erst_read' below will return "-ENOENT" value. This causes
+		 * 'retry_next' label is entered again. The returned value
+		 * should be zero indicating the read operation is EOF.
+		 */
+		len = 0;
+
 		goto out;
+	}
 retry:
 	rc = len = erst_read(id, erst_dbg_buf, erst_dbg_buf_len);
 	/* The record may be cleared by others, try read next record */