ext2fs: Fix the size of struct ufid 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 packed the structure and checks via a __Static_assert().

Approved by: so
Security: FreeBSD-SA-25:02.fs
Reviewed by: markj
MFC after: 2 weeks

(cherry picked from commit bfc8e3308bee23d0f7836d57f32ed8d47da02627)
(cherry picked from commit 7a3a0402aeb626a7379addd344cbfd8900e83baa)