NFS: Fix interrupted slots by sending a solo SEQUENCE operation

We used to do this before 3453d570, but this was changed to better
handle the NFS4ERR_SEQ_MISORDERED error code. This commit fixed the slot
re-use case when the server doesn't receive the interrupted operation,
but if the server does receive the operation then it could still end up
replying to the client with mis-matched operations from the reply cache.

We can fix this by sending a SEQUENCE to the server while recovering from
a SEQ_MISORDERED error when we detect that we are in an interrupted slot
situation.

Fixes: 3453d570 (NFSv4.1: Avoid false retries when RPC calls are interrupted)
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>