NFSv4: Simplify the struct nfs4_stateid

Replace the union with the common struct stateid4 as defined in both
RFC3530 and RFC5661. This makes it easier to access the sequence id,
which will again make implementing support for parallel OPEN calls
easier.

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