vmm: memory_manager: free memslot in remove_userspace_mapping

remove_userspace_mapping tears down the KVM mapping but never returns
the slot id to the allocator's free list. Call `free_memory_slot` to
avoid the leak.

Signed-off-by: Dylan Reid <dgreid@fb.com>

vmm/src/memory_manager.rs

@@ -2563,6 +2563,8 @@ impl MemoryManager {
             userspace_addr_ = userspace_addr as u64
         );
 
+        self.memory_slot_allocator().free_memory_slot(slot);
+
         Ok(())
     }