namecache: factor out ~MAKEENTRY lookups from the common path
Lookups of the sort are rare compared to regular ones and succesfull ones
result in removing entries from the cache.
In the current code buckets are rlocked and a trylock dance is performed,
which can fail and cause a restart. Fixing it will require a little bit
of surgery and in order to keep the code maintaineable the 2 cases have
to split.
MFC after: 1 week