bcachefs: Kill other unreachable() uses

Per previous commit, bare unreachable() considered harmful, convert to
BUG()

Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/bset.c

···
 	case BSET_RO_AUX_TREE:
 		return bset_search_tree(b, t, search, lossy_packed_search);
 	default:
-		unreachable();
+		BUG();
 	}
 }
 

fs/bcachefs/ec.h

···
 			bch2_ec_do_stripe_creates(c);
 			break;
 		default:
-			unreachable();
+			BUG();
 		}
 }