ocfs2: remove BUG_ON(!empty_extent) in __ocfs2_rotate_tree_left

Function ocfs2_rotate_tree_left call __ocfs2_rotate_tree_left for left rotation while non-rightmost path containing an empty extent in the leaf block. __ocfs2_rotate_tree_left return -EAGAIN if right subtree having an empty extent and pass the empty_extent_path to caller. The caller ocfs2_rotate_tree_left will restart rotation from the returned path.

It will trigger the BUG_ON(!ocfs2_is_empty_extent) when the et on disk is as follows:
eb0 is the leaf block of path(say path_a) passed to ocfs2_rotate_tree_left, which has an empty rec[0].
eb1 is the leaf block of path(say path_b) that just right to path_a, which has no empty record.
eb2 is the leaf block of path(say path_c) that just right to path_b, which has an empty rec[0]. And path_c is also the rightmost path.
Now we want to remove the empty rec[0] in eb0:
ocfs2_rotate_tree_left:
-> call __ocfs2_rotate_tree_left with path_a as its input *path*
-> call ocfs2_rotate_subtree_left with path_a as its input
*left_path* and path_b as its input *right_path*. it will move rec[0] in eb1 to eb0, and rec[0] in eb0 is not empty now.
-> continue to call ocfs2_rotate_subtree_left with path_b as its
input *left_path* and path_c as its input *right_path*, and return -EAGAIN because eb2 has an empty rec[0]
-> call __ocfs2_rotate_tree_left with path_c as it input, rotate all
records in eb2 to left and return 0.
-> call __ocfs2_rotate_tree_left with path_a as its input, and triggers
the BUG_ON(!ocfs2_is_empty_extent) as the rec[0] in eb0 is not empty.

So the BUG_ON() should be removed and return 0 if rec[0] is no longer an empty extent.

---
 fs/ocfs2/alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index ea4c822..5997c00 100644
@@ -2925,7 +2925,8 @@ static int __ocfs2_rotate_tree_left(handle_t *handle,
 	struct ocfs2_path *right_path = NULL;
 	struct super_block *sb = ocfs2_metadata_cache_get_super(et->et_ci);
 
-	BUG_ON(!ocfs2_is_empty_extent(&(path_leaf_el(path)->l_recs[0])));
+	if (!ocfs2_is_empty_extent(&(path_leaf_el(path)->l_recs[0])))
+		return 0;
 
 	*empty_extent_path = NULL;
 

1.8.4.3