csky: Fixup update_mmu_cache called with user io mapping

The function update_mmu_cache could be called by user-io mapping.
There is no space of struct page in mem_map for the pte. Just
ignore the user-io mmaping in update_mmu_cache.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

arch/csky/abiv2/cacheflush.c

@@ -12,6 +12,9 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
 	unsigned long addr;
 	struct page *page;
 
+	if (!pfn_valid(pte_pfn(*pte)))
+		return;
+
 	page = pfn_to_page(pte_pfn(*pte));
 	if (page == ZERO_PAGE(0))
 		return;