sh: Fix argument page dcache flushing regression.

In the do_execve() path, argument page handling used to explicitly call
flush_dcache_page() for each page, this has since been reworked and
uses flush_kernel_dcache_page() instead, which is presently a nop.

Doing a simple modprobe/rmmod in a loop under busybox consistently
manages to crash without providing a sane flush_kernel_dcache_page()
implementation, so, plug in a simple implementation.
Signed-off-by: NCarmelo Amoroso <carmelo73@gmail.com>
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

include/asm-sh/cacheflush.h

@@ -43,6 +43,12 @@ extern void __flush_purge_region(void *start, int size);
 extern void __flush_invalidate_region(void *start, int size);
 #endif
 
+#define ARCH_HAS_FLUSH_KERNEL_DCACHE_PAGE
+static inline void flush_kernel_dcache_page(struct page *page)
+{
+	flush_dcache_page(page);
+}
+
 #if defined(CONFIG_CPU_SH4) && !defined(CONFIG_CACHE_OFF)
 extern void copy_to_user_page(struct vm_area_struct *vma,
 	struct page *page, unsigned long vaddr, void *dst, const void *src,