vm_page: drop mpred param from insert_lookup

There is no longer a sorted page list maintained by a vm_object, so
there is no longer a need for the mpred parameter to
vm_page_insert_lookup(). Dropping that parameter creates new functions
that also don't need that parameter. Drop all those parameters. Some
of the functions have names ending in "_after", where "after" refers
to the mpred parameter that is being dropped. So that those names make
sense, change "_after" to "_iter" in all of them, because they all
take pctrie_iter parameters. Make those parameters last in parameter
lists, as "mpred" was before.

Reviewed by: alc, markj, kib
Differential Revision: https://reviews.freebsd.org/D50199