crypto: don't pollute the global namespace with sg_next()

It's a subsystem function, prefix it as such.

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/crypto/digest.c b/crypto/digest.c
index 1bf7414..e56de67 100644
--- a/crypto/digest.c
+++ b/crypto/digest.c
@@ -77,7 +77,7 @@
 
 		if (!nbytes)
 			break;
-		sg = sg_next(sg);
+		sg = scatterwalk_sg_next(sg);
 	}
 
 	return 0;