Convert allocations of the phys pager to vm_pager_allocate().

Future changes would require additional initialization of OBJT_PHYS
objects, and vm_object_allocate() is not suitable for it.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24652