btrfs: handle root deletion lookup error in btrfs_del_root()

We're deleting a root and looking it up by key does not succeed, this
is an inconsistent state and we can't do anything. All callers handle
errors and abort a transaction.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>