Btrfs: fix oops while writing data to SSD partitions

Here I have a two SSD-partitions btrfs, and they are defaultly set to
"data=raid0, metadata=raid1", then I try to fill my btrfs partition
till "No space left on device", via "dd if=/dev/zero of=/mnt/btrfs/tmp".

I get an oops panic from kernel BUG at fs/btrfs/extent-tree.c:5199!, which
refers to find_free_extent's
BUG_ON(index != get_block_group_index(block_group));

In SSD mode, in order to find enough space to alloc, we may check the
block_group cache which has been checked sometime before, but the index is not
updated, where it hits the BUG_ON.
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Acked-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -5079,7 +5079,9 @@ static noinline int find_free_extent(struct btrfs_trans_handle *trans,
 			 * group is does point to and try again
 			 */
 			if (!last_ptr_loop && last_ptr->block_group &&
-			    last_ptr->block_group != block_group) {
+			    last_ptr->block_group != block_group &&
+			    index <=
+				 get_block_group_index(last_ptr->block_group)) {
 
 				btrfs_put_block_group(block_group);
 				block_group = last_ptr->block_group;