bio: make freeing of ->bi_io_vec conditional in bio_free()

The empty barrier patches do not carry data, so they have no
iovec attached.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/bio.c

@@ -109,11 +109,14 @@ static inline struct bio_vec *bvec_alloc_bs(gfp_t gfp_mask, int nr, unsigned lon
 
 void bio_free(struct bio *bio, struct bio_set *bio_set)
 {
+	if (bio->bi_io_vec) {
 		const int pool_idx = BIO_POOL_IDX(bio);
 
 		BIO_BUG_ON(pool_idx >= BIOVEC_NR_POOLS);
 
 		mempool_free(bio->bi_io_vec, bio_set->bvec_pools[pool_idx]);
+	}
+
 	mempool_free(bio, bio_set->bio_pool);
 }