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>

arch/mips/include/asm/highmem.h

@@ -19,8 +19,10 @@
 
 #ifdef __KERNEL__
 
+#include <linux/bug.h>
 #include <linux/interrupt.h>
 #include <linux/uaccess.h>
+#include <asm/cpu-features.h>
 #include <asm/kmap_types.h>
 
 /* undef for production */
@@ -50,7 +52,7 @@ extern void *kmap_atomic(struct page *page);
 extern void __kunmap_atomic(void *kvaddr);
 extern void *kmap_atomic_pfn(unsigned long pfn);
 
-#define flush_cache_kmaps()	flush_cache_all()
+#define flush_cache_kmaps()	BUG_ON(cpu_has_dc_aliases)
 
 extern void kmap_init(void);