btrfs: remove BUG() after failure to insert delayed dir index item

In the Linux kernel, the following vulnerability has been resolved:

btrfs: remove BUG() after failure to insert delayed dir index item

Instead of calling BUG() when we fail to insert a delayed dir index item

into the delayed node's tree, we can just release all the resources we

have allocated/acquired before and return the error to the caller. This is

fine because all existing call chains undo anything they have done before

calling btrfs_insert_delayed_dir_index() or BUG_ON (when creating pending

snapshots in the transaction commit path).

So remove the BUG() call and do proper error handling.

This relates to a syzbot report linked below, but does not fix it because

it only prevents hitting a BUG(), it does not fix the issue where somehow

we attempt to use twice the same index number for different index items.