Make vm_pqbatch_submit_page() externally visible.

It will become useful for the page daemon to be able to directly create
a batch queue entry for a page, and without modifying the page
structure. Rename vm_pqbatch_submit_page() to vm_page_pqbatch_submit()
to keep the namespace consistent. No functional change intended.

Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21369