NFS add session back channel draining

Currently session draining only drains the fore channel.
The back channel processing must also be drained.

Use the back channel highest_slot_used to indicate that a callback is being
processed by the callback thread.  Move the session complete to be per channel.

When the session is draininig, wait for any current back channel processing
to complete and stop all new back channel processing by returning NFS4ERR_DELAY
to the back channel client.

Drain the back channel, then the fore channel.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index 1eaa054..e93ada0 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -197,6 +197,7 @@
 						 * op for dynamic resizing */
 	int		target_max_slots;	/* Set by CB_RECALL_SLOT as
 						 * the new max_slots */
+	struct completion complete;
 };
 
 static inline int slot_idx(struct nfs4_slot_table *tbl, struct nfs4_slot *sp)
@@ -213,7 +214,6 @@
 	unsigned long			session_state;
 	u32				hash_alg;
 	u32				ssv_len;
-	struct completion		complete;
 
 	/* The fore and back channel */
 	struct nfs4_channel_attrs	fc_attrs;