Fix broken rot90 userspace function

[Bogdan Mitrović]

My initial understanding of in-place square matrix rotation was flawed. Refer here for better understanding.

[Bogdan Mitrović]

A better approach for in-place rotation is to transpose the matrix first, and then reverse either the columns or the rows (depending on the rotation direction).

[Bogdan Mitrović]

Fixed in commit 13dcf34