Consistently use vm_ooffset_t type for the vm object offset in
elf_load_section.

The values passed currently as vm_offset_t are phdr.p_offset, which
have the native Elf word size. Since elf_load_section interprets them
as the file offset, use vm object offset type.

Noted and reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week