NFSv4.1: Handle NFS4ERR_BADSLOT errors correctly

Most (all) NFS4ERR_BADSLOT errors are due to the client failing to
respect the server's sr_highest_slotid limit. This mainly happens
due to reordered RPC requests.
The way to handle it is simply to drop the slot that we're using,
and retry using the new highest_slotid limits.

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