MIPS: highmem: Turn flush_cache_kmaps into a no-op.

MIPS: highmem: Turn flush_cache_kmaps into a no-op.

It was calling flush_cache_all() which is a no-op since a long time anyway
and which was overkill in the old days when it was actually doing something
because only the D-cache needs to be flushed, never the I-cache, never
the S-cache.  Since however highmem on MIPS is still only supported on
processors that don't suffer from cache aliases, we could turn
flush_cache_kmaps() into a no-op - but for paranoia's sake we rather make
it BUG_ON(cpu_has_dc_aliases()).

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