NFSv4.1: nfs4_callback_free_slot() cannot call nfs4_slot_tbl_drain_complete()

The current code works only for the case where we have exactly one slot,
which is no longer true.
nfs4_free_slot() will automatically declare the callback channel to be
drained when all slots have been returned.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>