btrfs: teach __process_pages_contig about PAGE_LOCK operation

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ changes to the helper separated from the following patch ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.h

@@ -45,13 +45,14 @@
 #define EXTENT_BUFFER_IN_TREE 10
 #define EXTENT_BUFFER_WRITE_ERR 11    /* write IO error */
 
-/* these are flags for extent_clear_unlock_delalloc */
+/* these are flags for __process_pages_contig */
 #define PAGE_UNLOCK		(1 << 0)
 #define PAGE_CLEAR_DIRTY	(1 << 1)
 #define PAGE_SET_WRITEBACK	(1 << 2)
 #define PAGE_END_WRITEBACK	(1 << 3)
 #define PAGE_SET_PRIVATE2	(1 << 4)
 #define PAGE_SET_ERROR		(1 << 5)
+#define PAGE_LOCK		(1 << 6)
 
 /*
  * page->private values.  Every page that is controlled by the extent