compress: replace compress_blob() with compress_blob_explicit()

And make compress_xyz() return 0 on success, as we know which compression
algorithm is used when calling compress_blob().

Follow-up for 2360352ef0.

src/fuzz/fuzz-compress.c

@@ -45,7 +45,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         }
 
         size_t csize;
-        r = compress_blob_explicit(alg, h->data, data_len, buf, size, &csize);
+        r = compress_blob(alg, h->data, data_len, buf, size, &csize);
         if (r < 0) {
                 log_error_errno(r, "Compression failed: %m");
                 return 0;