util: Refactor FileCommit from an #if sequence nested in #else, to a sequence of #elif

This should not change the actual code generation.

src/util/system.cpp

@@ -1024,8 +1024,7 @@ bool FileCommit(FILE *file)
         LogPrintf("%s: FlushFileBuffers failed: %d\n", __func__, GetLastError());
         return false;
     }
-#else
+#elif HAVE_FDATASYNC
-    #if HAVE_FDATASYNC
     if (fdatasync(fileno(file)) != 0 && errno != EINVAL) { // Ignore EINVAL for filesystems that don't support sync
         LogPrintf("%s: fdatasync failed: %d\n", __func__, errno);
         return false;
@@ -1041,7 +1040,6 @@ bool FileCommit(FILE *file)
         return false;
     }
 #endif
-#endif
     return true;
 }