bcachefs: Guard against unknown k.k->type in __bkey_invalid()

For forwards compatibility we have to allow unknown key types, and only
run the checks that make sense against them.

Fix a missing guard on k.k->type being known.

Reported-by:  <syzbot+ae4dc916da3ce51f284f@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>