Waiting on a btree node write with btree locks held can deadlock, if the
write errors: the write error path has to do do a btree update to drop
the pointer to the replica that errored.

The interior update path has to wait on in flight btree writes before
freeing nodes on disk. Previously, this was done in
bch2_btree_interior_update_will_free_node(), and could deadlock; now, we
just stash a pointer to the node and do it in
btree_update_nodes_written(), just prior to the transactional part of
the update.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>