vm_page: reset iterator after domainset drops lock

[dougm]

Summary

Restore the WAITFAIL check on resetting the iterator in vm_page_alloc_domain_iter. Modify vm_domainset_iter_page_init and vm_domainset_iter_page so that they can take an iterator argument and reset the iterator if vm_domainset_iter_releases and reacquires the object lock.

Test Plan

I repeated the graid1_6.sh stress test several times without failure. Without the ability to reset the iterator in vm_page_alloc_iter, this test fails.

[markj]

Rather than passing a flag around, can we pass the pctrie iterator to the domainset routines and have them reset the iterator when appropriate?

[pho]

I have been running tests with D50392.155617.patch for 13 hours without seeing any issues