NFSv4.1: Handle NFS4ERR_DELAY replies to OP_SEQUENCE correctly

Don't assume that the NFS4ERR_DELAY means that the server is processing
this slot id.

Fixes: 3453d570

 ("NFSv4.1: Avoid false retries when RPC calls are interrupted")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>