Fix slab name "biovec-(1<<(21-12))"

I'm getting a slab named "biovec-(1<<(21-12))". It is caused by unintended
expansion of the macro BIO_MAX_PAGES. This patch renames it to biovec-max.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org	# v4.14+
Signed-off-by: Jens Axboe <axboe@kernel.dk>