namecache: clean up struct namecache_ts handling

namecache_ts differs from mere namecache by few fields placed mid struct.
The access to the last element (the name) is thus special-cased.

The standard solution is to put new fields at the very beginning anad
embedd the original struct. The pointer shuffled around points to the
embedded part. If needed, access to new fields can be gained through
__containerof.

MFC after: 1 week