btrfs: deal with deletion errors when deleting delayed items

btrfs: deal with deletion errors when deleting delayed items

Currently, btrfs_delete_delayed_items() ignores any errors returned from
btrfs_batch_delete_items(). This looks fishy but it's not a problem at
the moment because:

1) Two of the errors returned from btrfs_batch_delete_items() are for
   impossible cases, cases where a delayed item does not match any item
   in the leaf the path points to - btrfs_delete_delayed_items() always
   calls btrfs_batch_delete_items() with a path that points to a leaf
   that contains an item matching a delayed item;

2) btrfs_batch_delete_items() may return an error from btrfs_del_items(),
   in which case it does not release the delayed items of the batch.

   At the moment this is harmless because btrfs_del_items() actually is
   always able to delete items, even if it returns an error - when it
   returns an error it's because it ended up with a leaf mostly empty
   (less than 1/3 full) and failed to migrate items from that leaf into
   its neighbour leaves - this is not critical, as all the items were
   deleted, we just left the tree a bit unbalanced, but it's still a
   valid tree and causes no harm, and future operations on the tree will
   eventually balance it.

   So even if we get an error from btrfs_del_items(), the delayed items
   will not be released but the next time we run delayed items we will
   find out, at btrfs_delete_delayed_items(), that they are not present
   in the tree anymore and then release them.

This is all a bit subtle, and it's certainly prone to be a disaster in
case btrfs_del_items() changes one day and may return errors before being
able to delete all the requested items, in which case we could leave the
filesystem in an inconsistent state as we would commit a transaction
despite a failure from deleting items from the tree.

So make btrfs_delete_delayed_items() check for any errors from the call
to btrfs_batch_delete_items().

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>