[MIPS] Fix aliasing bug in copy_user_highpage.

Copy_user_highpage was written assuming it was only being called for
breaking COW pages in which case the source page isn't cached as in
marked cachable under it kernel virtual address.  If it is called anyway
the aliasing avoidance strategy implemented by kmap_coherent will fail.
Avoid the use of kmap_coherent for pages marked dirty and to avoid
another instance of this sort of bug, place a BUG_ON in kmap_coherent.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>