Bugfix: Make alloc_at return the start of the virtual memory range instead of the end

kernel/src/memory/MemoryManager.cpp

@@ -190,6 +190,8 @@ namespace MemoryManager
     {
         CHECK_PAGE_ALIGNED(virt);
 
+        u64 start = virt;
+
         while (count--)
         {
             u64 frame = TRY(alloc_frame());
@@ -197,7 +199,7 @@ namespace MemoryManager
             virt += ARCH_PAGE_SIZE;
         }
 
-        return virt;
+        return start;
     }
 
     Result<void> unmap_owned(u64 virt, usize count)