Introduce vm_page_alloc_noobj_contig()

This is the same as vm_page_alloc_noobj(), but allocates physically
contiguous runs of memory. For now it is implemented in terms of
vm_page_alloc_contig(), with the difference that
vm_page_alloc_noobj_contig() implements VM_ALLOC_ZERO by zeroing the
page.

Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 92db9f3bb7623883231214e74ec38788c3dffc6a)