NFS: Clean up nfs_sb_active/nfs_sb_deactive

Instead of causing umount requests to block on server->active_wq while the
asynchronous sillyrename deletes are executing, we can use the sb->s_active
counter to obtain a reference to the super_block, and then release that
reference in nfs_async_unlink_release().

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

include/linux/nfs_fs_sb.h

@@ -119,7 +119,6 @@ struct nfs_server {
 	void (*destroy)(struct nfs_server *);
 
 	atomic_t active; /* Keep trace of any activity to this server */
-	wait_queue_head_t active_wq;  /* Wait for any activity to stop  */
 
 	/* mountd-related mount options */
 	struct sockaddr_storage	mountd_address;