follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
target of FOLL_WRITE either. However add the checks.
Namely, either the need to CoW due to missing write bit, or proper CoR on
!AnonExclusive pages over R/O pins to reject the follow page. That brings
this function closer to follow_hugetlb_page().
So we don't care before, and also for now. But we'll care if we switch
over slow-gup to use hugetlb_follow_page_mask(). We'll also care when to
return -EMLINK properly, as that's the gup internal api to mean "we should
do CoR". Not really needed for follow page path, though.
When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
clear that it just should never fail.