Fix zero length sys_cacheflush

Cacheflush(0, 0, 0) was crashing the system.  This is because
flush_icache_range(start, end) tries to flushing whole address space
(0 - ~0UL) if both start and end are zero.
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/cache.c

@@ -56,6 +56,8 @@ EXPORT_SYMBOL(_dma_cache_inv);
 asmlinkage int sys_cacheflush(unsigned long __user addr,
 	unsigned long bytes, unsigned int cache)
 {
+	if (bytes == 0)
+		return 0;
 	if (!access_ok(VERIFY_WRITE, (void __user *) addr, bytes))
 		return -EFAULT;