bcachefs: Kill other unreachable() uses

Per previous commit, bare unreachable() considered harmful, convert to
BUG()
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bset.c

@@ -1189,7 +1189,7 @@ struct bkey_packed *__bch2_bset_search(struct btree *b,
 	case BSET_RO_AUX_TREE:
 		return bset_search_tree(b, t, search, lossy_packed_search);
 	default:
-		unreachable();
+		BUG();
 	}
 }
 

fs/bcachefs/ec.h

@@ -240,7 +240,7 @@ static inline void ec_stripe_new_put(struct bch_fs *c, struct ec_stripe_new *s,
 			bch2_ec_do_stripe_creates(c);
 			break;
 		default:
-			unreachable();
+			BUG();
 		}
 }