reference: https://github.com/koverstreet/bcachefs/issues/692

trans->ref is the reference used by the cycle detector, which walks
btree_trans objects of other threads to walk the graph of held locks and
issue wakeups when an abort is required.

We have to wait for the ref to go to 1 before freeing trans->paths or
clearing trans->locking_wait.task.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>