NFSv4: Avoid thundering herd issues with nfs_release_seqid

Store a pointer to the rpc_task in struct nfs_seqid so that we can wake up
only that request that is able to grab the lock after we've released it.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/nfs4_fs.h b/fs/nfs/nfs4_fs.h
index c45c21a..b133b50 100644
--- a/fs/nfs/nfs4_fs.h
+++ b/fs/nfs/nfs4_fs.h
@@ -71,6 +71,7 @@
 struct nfs_seqid {
 	struct nfs_seqid_counter *sequence;
 	struct list_head list;
+	struct rpc_task *task;
 };
 
 static inline void nfs_confirm_seqid(struct nfs_seqid_counter *seqid, int status)