bcachefs: Correctly validate k->u64s in btree node read path

validate_bset_keys() never properly validated k->u64s; it checked if it
was 0, but not if it was smaller than keys for the given packed format;
this fixes that small oversight.

This patch was backported, so it's adding quite a few error enums so
that they don't get renumbered and we don't have confusing gaps.

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