tarfs: Fix the size of struct tarfs_fid and add a static assert

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().

Reviewed by: markj
MFC after: 2 weeks

(cherry picked from commit 4db1b113b15158c7d134df83e7a7201cf46d459b)