fs/bio: remove bs paramater in biovec_create_pool

bs is no longer used in biovec_create_pool since 9f060e22 ("block:
Convert integrity to bvec_alloc_bs()")

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@fb.com>