vm_domainset_iter_first() would not check if the initial domain selected
by the policy was effectively valid (i.e., allowed by the domainset and
not marked as ignored by vm_domainset_iter_ignore()). It would just try
to skip it if it had less pages than 'free_min', and would not take into
account the possibility of no domains being valid.
Factor out code that logically belongs to the iterator machinery and is
not tied to how allocations (or impossibility thereof) are to be
handled. This allows to remove duplicated code between
vm_domainset_iter_page() and vm_domainset_iter_policy(), and between
vm_domainset_iter_page_init() and _vm_domainset_iter_policy_init().
This also allows to remove the 'pages' parameter from
vm_domainset_iter_page_init().
This also makes the two-phase logic clearer, revealing an inconsistency
between setting 'di_minskip' to true in vm_domainset_iter_init()
(implying that, in the case of waiting allocations, further attempts
after the first sleep should just allocate for the first domain,
regardless of their situation with respect to their 'free_min') and
trying to skip the first domain if it has too few pages in
vm_domainset_iter_page_init() and _vm_domainset_iter_policy_init(). Fix
this inconsistency by resetting 'di_minskip' to 'true' in
vm_domainset_iter_first() instead so that, after each vm_wait_doms()
(waiting allocations that could not be satisfied immediately), we again
start with only the domains that have more than 'free_min' pages.
While here, fix the minor quirk that the round-robin policy would start
with the domain after the one pointed to by the initial value of
'di_iter' (this just affects the case of resetting '*di_iter', and would
not cause domain skips in other circumstances, i.e., for waiting
allocations that actually wait or at each subsequent new iterator
creation with same iteration index storage).