NFSv4.1: Fix an ABBA locking issue with session and state serialisation

Ensure that if nfs_wait_on_sequence() causes our rpc task to wait for
an NFSv4 state serialisation lock, then we also drop the session slot.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org