bcachefs: cryptographic MACs on superblock are not (yet?) supported

bcachefs: cryptographic MACs on superblock are not (yet?) supported

We should add support for cryptographic macs on the superblock - and it
won't be hard, but it'll need an incompatible feature bit (and we have a
new incompatible feature versioning scheme coming).

For now, just add a guard to avoid a dull ptr deref in gen_poly_key().

Reported-by: syzbot+dd3d9835055dacb66f35@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>