Fix bio_alloc() and bio_kmalloc() documentation

Commit 451a9ebf accidentally broke bio_alloc() and bio_kmalloc() comments by
(almost) swapping them.

This patch fixes that, by placing the comments in the right place.

Signed-off-by: Alberto Bertogli <albertito@blitiri.com.ar>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>