ocfs2: remove BUG_ON(!empty_extent) in __ocfs2_rotate_tree_left()

ocfs2_rotate_tree_left() calls __ocfs2_rotate_tree_left() for left
rotation while non-rightmost path containing an empty extent in the leaf
block.  __ocfs2_rotate_tree_left() returns -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.
Signed-off-by: joyce.xue <xuejiufei@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ocfs2/alloc.c

@@ -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;