Fix image reuse with multi-memory images (#3846)

This commit fixes a potential issue where the fast-path instantiate in
`MemoryImageSlot` where when the previous image is compared against the
new image it only performed file descriptor equality, but nowadays with
loading images from `*.cwasm` files there might be multiple images in
the same file so the offsets also need to be considered. I think this
isn't really easy to hit today, it would require combining both module
linking and multi-memory which gets into the realm of being pretty
esoteric so I haven't added a test case here for this.

crates/runtime/src/cow.rs

@@ -27,7 +27,7 @@ impl ModuleMemoryImages {
 }
 
 /// One backing image for one memory.
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub struct MemoryImage {
     /// The file descriptor source of this image.
     ///
@@ -75,6 +75,12 @@ impl FdSource {
     }
 }
 
+impl PartialEq for FdSource {
+    fn eq(&self, other: &FdSource) -> bool {
+        self.as_file().as_raw_fd() == other.as_file().as_raw_fd()
+    }
+}
+
 impl MemoryImage {
     fn new(
         page_size: u32,
@@ -348,14 +354,7 @@ impl MemoryImageSlot {
         // termination. The `clear_and_remain_ready()` path also
         // mprotects memory above the initial heap size back to
         // PROT_NONE, so we don't need to do that here.
-        if (self.image.is_none()
-            && maybe_image.is_none()
-            && self.initial_size == initial_size_bytes)
-            || (self.image.is_some()
-                && maybe_image.is_some()
-                && self.image.as_ref().unwrap().fd.as_file().as_raw_fd()
-                    == maybe_image.as_ref().unwrap().fd.as_file().as_raw_fd())
-        {
+        if self.image.as_ref() == maybe_image && self.initial_size == initial_size_bytes {
             self.dirty = true;
             return Ok(());
         }