vm_page: move tailq validation to grab_pages

Function vm_page_acquire_unlocked both looks up pages and validates
them. Much of it serves the needs of only one caller,
vm_page_grab_pages_unlocked, by checking the validity of checking
tailq links. Extract from that function the parts that serve only
vm_page_grab_pages_unlocked, and move them into that function.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D47001