Relax the object type restrictions on vm_page_alloc_contig(). Specifically,
add support for object types that were previously prohibited because they
could contain PG_CACHED pages.

Roughly halve the number of radix trie operations performed by
vm_page_alloc_contig() using the same approach that is employed by
vm_page_alloc(). Also, eliminate the radix trie lookup performed with the
free page queues lock held.

Tidy up the handling of radix trie insert failures in vm_page_alloc() and
vm_page_alloc_contig().

Reviewed by: kib, markj
Tested by: pho
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8878