block: Remove bi_idx references

For immutable bvecs, all bi_idx usage needs to be audited - so here
we're removing all the unnecessary uses.

Most of these are places where it was being initialized on a bio that
was just allocated, a few others are conversions to standard macros.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>

fs/buffer.c

@@ -2979,7 +2979,6 @@ int submit_bh(int rw, struct buffer_head * bh)
 	bio->bi_io_vec[0].bv_offset = bh_offset(bh);
 
 	bio->bi_vcnt = 1;
-	bio->bi_idx = 0;
 	bio->bi_size = bh->b_size;
 
 	bio->bi_end_io = end_bio_bh_io_sync;