NFSv41: Fix a potential state leakage when restarting nfs4_close_prepare

Currently, if the call to nfs4_setup_sequence() in nfs4_close_prepare
fails, any later retries will fail to launch an RPC call, due to the fact
that the &state->flags will have been cleared.
Ditto if nfs4_close_done() triggers a call to the NFSv4.1 version of
nfs_restart_rpc().

We therefore move the actual clearing of the state->flags to
nfs4_close_done(), when we know that the RPC call was successful.

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