vm_page: expose page_alloc_after

vm_page_alloc() just calls vm_page_alloc_after(), after it has found
the predecessor of a page parameter. Many callers of vm_page_alloc()
already know that predecessor. Letting them pass that to
vm_page_alloc_after() directly could save a little redundant
calculation.

Reviewed by: alc
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D49103