iov_iter: unify iterate_iovec and iterate_kvec

The differences between iterate_iovec and iterate_kvec are minor:
	* kvec callback is treated as if it returned 0
	* initialization of __p is with i->iov and i->kvec resp.
which is trivially dealt with.

No code generation changes - compiler is quite capable of turning
	left = ((void)(STEP), 0);
	__v.iov_len -= left;
(with no accesses to left downstream) and
	(void)(STEP);
into the same code.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -16,10 +16,10 @@
 
 #define PIPE_PARANOIA /* for now */
 
+/* covers iovec and kvec alike */
 #define iterate_iovec(i, n, __v, __p, skip, STEP) {		\
 	size_t left;						\
 	size_t wanted = n;					\
-	__p = i->iov;						\
 	do {							\
 		__v.iov_len = min(n, __p->iov_len - skip);	\
 		if (likely(__v.iov_len)) {			\
@@ -37,25 +37,6 @@
 	n = wanted - n;						\
 }
 
-#define iterate_kvec(i, n, __v, __p, skip, STEP) {		\
-	size_t wanted = n;					\
-	__p = i->kvec;						\
-	do {							\
-		__v.iov_len = min(n, __p->iov_len - skip);	\
-		if (likely(__v.iov_len)) {			\
-			__v.iov_base = __p->iov_base + skip;	\
-			(void)(STEP);				\
-			skip += __v.iov_len;			\
-			n -= __v.iov_len;			\
-			if (skip < __p->iov_len)		\
-				break;				\
-		}						\
-		__p++;						\
-		skip = 0;					\
-	} while (n);						\
-	n = wanted - n;						\
-}
-
 #define iterate_bvec(i, n, __v, __bi, skip, STEP) {	\
 	struct bvec_iter __start;			\
 	__start.bi_size = n;				\
@@ -109,7 +90,7 @@
 	if (likely(n)) {					\
 		size_t skip = i->iov_offset;			\
 		if (likely(iter_is_iovec(i))) {			\
-			const struct iovec *iov;		\
+			const struct iovec *iov = i->iov;	\
 			struct iovec v;				\
 			iterate_iovec(i, n, v, iov, skip, (I))	\
 			i->nr_segs -= iov - i->iov;		\
@@ -123,9 +104,10 @@
 			i->nr_segs -= i->bvec - bvec;		\
 			skip = __bi.bi_bvec_done;		\
 		} else if (iov_iter_is_kvec(i)) {		\
-			const struct kvec *kvec;		\
+			const struct kvec *kvec = i->kvec;	\
 			struct kvec v;				\
-			iterate_kvec(i, n, v, kvec, skip, (K))	\
+			iterate_iovec(i, n, v, kvec, skip,	\
+						((void)(K),0))	\
 			i->nr_segs -= kvec - i->kvec;		\
 			i->kvec = kvec;				\
 		} else if (iov_iter_is_xarray(i)) {		\