nfsd: Convert idmap to use kuids and kgids

Convert nfsd_map_name_to_uid to return a kuid_t value.
Convert nfsd_map_name_to_gid to return a kgid_t value.
Convert nfsd_map_uid_to_name to take a kuid_t parameter.
Convert nfsd_map_gid_to_name to take a kgid_t paramater.

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>