NFSv4: Change the default setting of the nfs4_disable_idmapping parameter

NFSv4: Change the default setting of the nfs4_disable_idmapping parameter

Now that the use of numeric uids/gids is officially sanctioned in
RFC3530bis, it is time to change the default here to 'enabled'.

By doing so, we ensure that NFSv4 copies the behaviour of NFSv3 when we're
using the default AUTH_SYS authentication (i.e. when the client uses the
numeric uids/gids as authentication tokens), so that when new files are
created, they will appear to have the correct user/group.
It also fixes a number of backward compatibility issues when migrating
from NFSv3 to NFSv4 on a platform where the server uses different uid/gid
mappings than the client.

Note also that this setting has been successfully tested against servers
that do not support numeric uids/gids at several Connectathon/Bakeathon
events at this point, and the fall back to using string names/groups has
been shown to work well in all those test cases.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>