iov_iter_bvec_advance(): don't bother with bvec_iter

do what we do for iovec/kvec; that ends up generating better code,
AFAICS.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -846,17 +846,22 @@ static void pipe_advance(struct iov_iter *i, size_t size)
 
 static void iov_iter_bvec_advance(struct iov_iter *i, size_t size)
 {
-	struct bvec_iter bi;
+	const struct bio_vec *bvec, *end;
 
-	bi.bi_size = i->count;
-	bi.bi_bvec_done = i->iov_offset;
-	bi.bi_idx = 0;
-	bvec_iter_advance(i->bvec, &bi, size);
+	if (!i->count)
+		return;
+	i->count -= size;
+
+	size += i->iov_offset;
 
-	i->bvec += bi.bi_idx;
-	i->nr_segs -= bi.bi_idx;
-	i->count = bi.bi_size;
-	i->iov_offset = bi.bi_bvec_done;
+	for (bvec = i->bvec, end = bvec + i->nr_segs; bvec < end; bvec++) {
+		if (likely(size < bvec->bv_len))
+			break;
+		size -= bvec->bv_len;
+	}
+	i->iov_offset = size;
+	i->nr_segs -= bvec - i->bvec;
+	i->bvec = bvec;
 }
 
 static void iov_iter_iovec_advance(struct iov_iter *i, size_t size)