nfs3: fix readdir entry size calculation

Motivation:
when estimating readdir reply size we should not forget about boolean
flag that indicates the presence of a next entry.

Modification:
fix entry overhead size.

Result:
The returned xdr doesn't overflow expected reply size.

Acked-by: Paul Millar
Target: master, 0.22
1 job for master in 1 minute and 20 seconds (queued for 24 seconds)
Status Name Job ID Coverage
  Build
passed build #8477

00:01:20