This is a bugfix for setkeycode() in /drivers/char/keyboard.c.

If we change a keycode the corresponding bit should be cleared if and
only if this keycode is not defined any longer.  I believe that this
also was intended with the original code, but the implementation is
faulty.

First off all the first three changed lines are obviously erroneus:
oldkey == truekey is false or true, you do not need to inclose this in a
for().  I believe the author intended INPUT_KEYCODE(dev,i) == oldkey.
But fixing this alone is not enough.

If somebody wants to interchange the definition of two keys A and B, the
normal way is to use two setkeycode calls:

    setkeycode (scancode A, keycode B);
    setkeycode (scancode B, keycode A);

The old code does a clearbit(oldkey ..) call even in situations where
two keys have the same definition, and this situation arises commonly in
the situation mentioned above.

Both errors are fixed with this patch.