async_tx: use memchr_inv

Use memchr_inv() to check the specified page is filled with zero.
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Dan Williams <djbw@fb.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

crypto/async_tx/async_xor.c

@@ -230,9 +230,7 @@ EXPORT_SYMBOL_GPL(async_xor);
 
 static int page_is_zero(struct page *p, unsigned int offset, size_t len)
 {
-	char *a = page_address(p) + offset;
-	return ((*(u32 *) a) == 0 &&
-		memcmp(a, a + 4, len - 4) == 0);
+	return !memchr_inv(page_address(p) + offset, 0, len);
 }
 
 static inline struct dma_chan *