cd9660: Make sure that struct ifid fits in generic filehandle structure

File system specific *fid structures are copied into the generic
struct fid defined in sys/mount.h.
As such, they cannot be larger than struct fid.

This patch packs the structure and checks via a __Static_assert().

Reported by: Kevin Miller <mas@0x194.net>
Reviewed by: olce, imp, kib, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47879