Kernel: Call did_modify_contents() after truncate()

Truncating a file should absolutly mark it as modified. With this
change our `tail` implementation can correctly handle it.

Kernel/FileSystem/Ext2FS/Inode.cpp

@@ -1078,6 +1078,7 @@ ErrorOr<void> Ext2FSInode::truncate(u64 size)
         return {};
     TRY(resize(size));
     set_metadata_dirty(true);
+    did_modify_contents();
     return {};
 }