NFSv4.1: Clean up nfs4_free_slot

Change the argument to take the pointer to the slot, instead of
just the slotid.

We know that the new value of highest_used_slot must be less than
the current value. No need to scan the whole table.

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