bcachefs: bch2_checksum() returns 0 for unknown checksum type

This fixes missing guards on trying to calculate a checksum with an
invalid/unknown checksum type; moving the guards up to e.g. btree_io.c
might be "more correct", but doesn't buy us anything - an unknown
checksum type will always be flagged as at least a checksum error so we
aren't losing any safety doing it this way and it makes it less likely
to accidentally pop an assert we don't want.

Reported-by: syzbot+e951ad5349f3a34a715a@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/checksum.c

@@ -233,7 +233,7 @@ struct bch_csum bch2_checksum(struct bch_fs *c, unsigned type,
 		return ret;
 	}
 	default:
-		BUG();
+		return (struct bch_csum) {};
 	}
 }
 
@@ -307,7 +307,7 @@ static struct bch_csum __bch2_checksum_bio(struct bch_fs *c, unsigned type,
 		return ret;
 	}
 	default:
-		BUG();
+		return (struct bch_csum) {};
 	}
 }