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.

(cherry picked from commit d00c64bb2347cc620d31a178c7755aa7e594f065)