Factor out the kmem contig page alloc and reclamation code.

kmem_alloc_attr_domain() and kmem_alloc_contig_domain() duplicated each
other's page allocation and reclamation logic. Place it in a single
function to make it easier to add additional consumers. No functional
change intended.

Reviewed by: jeff, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24475