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.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25602