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>
diff --git a/fs/nfs/callback_xdr.c b/fs/nfs/callback_xdr.c
index c14758e..390ac9c 100644
--- a/fs/nfs/callback_xdr.c
+++ b/fs/nfs/callback_xdr.c
@@ -753,7 +753,6 @@
 	 * A single slot, so highest used slotid is either 0 or -1
 	 */
 	nfs4_free_slot(tbl, slot);
-	nfs4_slot_tbl_drain_complete(tbl);
 	spin_unlock(&tbl->slot_tbl_lock);
 }