uefi: report remaining memory map entries

ExactSizeIterator::len must return the number of items still available.
Account for the iterator index after entries have been consumed.

uefi/src/mem/memory_map/impl_.rs

@@ -513,6 +513,10 @@ mod tests {
             mmap.entries().copied().collect::<Vec<_>>().as_slice(),
             &BASE_MMAP_UNSORTED
         );
+        let mut entries = mmap.entries();
+        assert_eq!(entries.len(), 3);
+        assert!(entries.next().is_some());
+        assert_eq!(entries.len(), 2);
         assert!(!mmap.is_sorted());
     }
 

uefi/src/mem/memory_map/iter.rs

@@ -33,6 +33,6 @@ impl<'a> Iterator for MemoryMapIter<'a> {
 
 impl ExactSizeIterator for MemoryMapIter<'_> {
     fn len(&self) -> usize {
-        self.memory_map.len()
+        self.memory_map.len() - self.index
     }
 }