Fix a bug that causes uvm_pmr_get1page() to fail for allocations that

Fix a bug that causes uvm_pmr_get1page() to fail for allocations that
specify an address constraint even when free pages that meet the constraint
are still available.  This happens because the old code was using the root
of the size tree as a starting point for a search down the address tree.
This meant only part of the address tree was searched, and that part could
very well not contain any of the pages that met the constraint.  Instead,
always walk the address tree from its root if the list of single pages is
empty and the root of the size tree doesn't meet our constraints.

From Visa Hankala.

ok deraadt@