bcachefs: Fix a race in btree_update_nodes_written()

One btree update might have terminated in a node update, and then while
it is in flight another btree update might free that original node.

This race has to be handled in btree_update_nodes_written() - we were
missing a READ_ONCE().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>