block: Kill bi_destructor

Now that we've got generic code for freeing bios allocated from bio
pools, this isn't needed anymore.

This patch also makes bio_free() static, since without bi_destructor
there should be no need for it to be called anywhere else.

bio_free() is now only called from bio_put, so we can refactor those a
bit - move some code from bio_put() to bio_free() and kill the redundant
bio->bi_next = NULL.

v5: Switch to BIO_KMALLOC_POOL ((void *)~0), per Boaz
v6: BIO_KMALLOC_POOL now NULL, drop bio_free's EXPORT_SYMBOL
v7: No #define BIO_KMALLOC_POOL anymore

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-core.c b/block/blk-core.c
index 95c4935..b776cc9 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2807,7 +2807,7 @@
 
 free_and_out:
 	if (bio)
-		bio_free(bio, bs);
+		bio_put(bio);
 	blk_rq_unprep_clone(rq);
 
 	return -ENOMEM;