Btrfs: write_extent_pages came in 2.6.23

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -2107,7 +2107,7 @@ done:
 	return 0;
 }
 
-#if LINUX_VERSION_CODE <= KERNEL_VERSION(2,6,20)
+#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,23)
 /* Taken directly from 2.6.23 for 2.6.18 back port */
 typedef int (*writepage_t)(struct page *page, struct writeback_control *wbc,
                                 void *data);