NFS: Fall back on old idmapper if request_key() fails
This patch removes the CONFIG_NFS_USE_NEW_IDMAPPER compile option.
First, the idmapper will attempt to map the id using /sbin/request-key
and nfsidmap. If this fails (if /etc/request-key.conf is not configured
properly) then the idmapper will call the legacy code to perform the
mapping. I left a comment stating where the legacy code begins to make
it easier for somebody to remove in the future.