NFSv4.1: Add a helper function to deal with expired stateids

In NFSv4.1 and newer, if the server decides to revoke some or all of
the protocol state, the client is required to iterate through all the
stateids that it holds and call TEST_STATEID to determine which stateids
still correspond to valid state, and then call FREE_STATEID on the
others.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Tested-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>