block: fix memory leak of bvec

bio_init() clears bio instance, so the bvec index has to be set after
bio_init(), otherwise bio->bi_io_vec may be leaked.

Fixes: 3175199a ("block: split bio_kmalloc from bio_alloc_bioset")
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -482,8 +482,8 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, unsigned int nr_iovecs,
 		if (unlikely(!bvl))
 			goto err_free;
 
-		bio->bi_flags |= idx << BVEC_POOL_OFFSET;
 		bio_init(bio, bvl, bvec_nr_vecs(idx));
+		bio->bi_flags |= idx << BVEC_POOL_OFFSET;
 	} else if (nr_iovecs) {
 		bio_init(bio, bio->bi_inline_vecs, BIO_INLINE_VECS);
 	} else {