Btrfs: fix extent buffer leak after backref walking

commit 47fb091f(Btrfs: fix unlock after free on rewinded tree blocks)
takes an extra increment on the reference of allocated dummy extent buffer, so now we
cannot free this dummy one, and end up with extent buffer leak.
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Reviewed-by: NJan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/ctree.c

@@ -1271,7 +1271,6 @@ tree_mod_log_rewind(struct btrfs_fs_info *fs_info, struct extent_buffer *eb,
 		BUG_ON(!eb_rewin);
 	}
 
-	extent_buffer_get(eb_rewin);
 	btrfs_tree_read_unlock(eb);
 	free_extent_buffer(eb);