MFC r363062: shmfd: make shm_size a vm_ooffset_t

On 32-bit platforms, this expands the shm_size to a 64-bit quantity and
resolves a mismatch between the shmfd size and underlying vm_object size.
The implementation did not account for this kind of mismatch.

For compatibility's sake, the old shm_size has been replaced with
shm_oldsize (still popualted) and the new member appended to the end.