bcachefs: Make sure to initialize equiv when creating new snapshots

bcachefs: Make sure to initialize equiv when creating new snapshots

Previously, equiv was set in the snapshot deletion path, which is where
it's needed - equiv, for snapshot ID equivalence classes, would ideally
be a private data structure to the snapshot deletion path.

But if a new snapshot is created while snapshot deletion is running,
move_key_to_correct_snapshot() moves a key to snapshot id 0 - oops.

Fixes: https://github.com/koverstreet/bcachefs/issues/593
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>