Set uninitialized popmap bits in vm_reserv_init

In vm_reserv_init, set all the marker popmap bits in vm_reserv_init,
and not just the bits of the first popmap entry.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33258

(cherry picked from commit 9f32cb5b1c81ceed28491c031cd109811a5f5d7a)