NFSD: Delay adding new entries to LRU

Neil Brown observes:
> I would not include RC_INPROG entries in the lru at all - they are
> always ignored, and will be added when they are switched to
> RCU_DONE.

I also removed a stale comment.

Suggested-by: NeilBrown <neil@brown.name>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>