vm_page: alloc_domain_iter

A recent change that made a call to pctrie_iter_reset conditional was
ill-advised, as it leads to an assertion failure on the graid1_6.sh
stress2 test. Make it unconditional again.

Reported by: pho
Fixes: 3fe2f81f665b ("vm_page: reduce iter_reset calls in alloc_domain_iter")