loader: fix the size of MODINFOMD_MODULEP

The data in MODINFOMD_MODULEP is packed by the loader as a 4 byte type, but
the amd64 kernel expects a vm_paddr_t, which is of size 8 bytes. Fix this by
saving it as 8 bytes in the loader and retrieving it using the proper type
in the kernel.

Sponsored by: Citrix Systems R&D