nfs: fall back to numeric mapping by default

Linux client uses number based mapping by default.
This makes dCache default configuration to have
'bad' behaviour. Let make our default same as linux
has.

Acked-by: Paul Millar
Target: master, 2.6
Require-book: no
Require-notes: yes
(cherry picked from commit 2d6a1aa51288a4051408b06b467d80b1422ebca9)