Pooling allocator: Default for allocation policy should use memfd feature, not memfd-allocator. (#3777)

Thanks to @peterheune for noticing this!

crates/runtime/src/instance/allocator/pooling.rs

@@ -259,16 +259,14 @@ pub enum PoolingAllocationStrategy {
 }
 
 impl Default for PoolingAllocationStrategy {
-    #[cfg(feature = "memfd-allocator")]
     fn default() -> Self {
+        if cfg!(memfd) {
             Self::ReuseAffinity
-    }
-
-    #[cfg(not(feature = "memfd-allocator"))]
-    fn default() -> Self {
+        } else {
             Self::NextAvailable
         }
     }
+}
 
 /// Represents a pool of maximal `Instance` structures.
 ///