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
    
    (cherry picked from commit 205659c43d87bd42c4a0819fde8f81e8ebba068e)