contig allocs: Don't retry forever on M_WAITOK.

This restores behavior from before domain iterators were added in
r327895 and r327896.

The vm_domainset_iter_policy() will do a vm_wait_doms() and then
restart its iterator when M_WAITOK is set. It will also force
the containing loop to have M_NOWAIT. So we get an unbounded
retry loop rather than the intended bounded retries that
kmem_alloc_contig_pages() already handles.

This also restores M_WAITOK to the vmem_alloc() call in
kmem_alloc_attr_domain() and kmem_alloc_contig_domain().

Reviewed by: markj, kib
MFC after: 2 weeks
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D27507