Apparently, both nfs clients do not use cache_enter_time()
consistently, creating some namecache entries without NCF_TS flag.
This causes panic due to failed assertion.

As a temporal relief, remove the assert. Return epoch timestamp for
the entries without timestamp if asked.

While there, consolidate the code which returns timestamps, into a
helper cache_out_ts().

Discussed with: jhb
MFC after: 2 weeks