NFSv4: Convert the NFS client idmapper to use the container user namespace

When mapping NFS identities using the NFSv4 idmapper, we want to substitute
for the uids and gids that would normally go on the wire as part of a
NFSv3 request. So we use the same mapping in the NFSv4 upcall as we
use in the NFSv3 RPC call (i.e. the mapping stored in the rpc_clnt cred).

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>