nfscl: Purge name cache when readdir_plus is done

nfscl: Purge name cache when readdir_plus is done

The author reported that this patch was needed to avoid
crashes on a fairly busy RISC-V system.  The author did not
provide details w.r.t. the crashes.  Although I
have not seen any such crash, the patch looks reasonable
and I have not found any regressions when testing it.

Since "rdirplus" is not a default option, the patch is
only needed if you are doing NFS mounts with the "rdirplus"
mount option and seeing crashes related to the name cache.

MFC after:	1 week