ext2fs: 'struct ufid': Re-order fields and unpack

Re-ordering the fields suppresses the trailing padding which was causing
the structure to overflow 'struct fid'.

While here, re-indent in a more visually pleasing way.

Reviewed by: rmacklem, emaste, markj
Approved by: markj (mentor)
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D47955