LinuxKPI: add dev_alloc_pages()

Add dev_alloc_pages() though it seems a weird KPI, not passing a dev
despite its name. Used by updated wireless driver.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D44588

(cherry picked from commit 738c02ba24c66391870067602f1c9c030d5c5e5d)