block: Convert various code to bio_for_each_segment()

With immutable biovecs we don't want code accessing bi_io_vec directly -
the uses this patch changes weren't incorrect since they all own the
bio, but it makes the code harder to audit for no good reason - also,
this will help with multipage bvecs later.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Cc: Joern Engel <joern@logfs.org>
Cc: Prasad Joshi <prasadjoshi.linux@gmail.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index d488f80..a31e4da 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -65,9 +65,9 @@
 {
 	int i;
 	int error = !test_bit(BIO_UPTODATE, &bio->bi_flags);
+	struct bio_vec *bvec;
 
-	for (i = 0; i < bio->bi_vcnt; i++) {
-		struct bio_vec *bvec = &bio->bi_io_vec[i];
+	bio_for_each_segment_all(bvec, bio, i) {
 		struct page *page = bvec->bv_page;
 		struct buffer_head *bh, *head;
 		unsigned bio_start = bvec->bv_offset;