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
(cherry picked from commit ea6f71a3)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
1 job for 0.22 in 1 minute and 12 seconds (queued for 19 seconds)