NFS: Convert lookups of the open context to RCU

Reduce contention on the inode->i_lock by ensuring that we use RCU
when looking up the NFS open context.

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

include/linux/nfs_fs.h

@@ -83,6 +83,7 @@ struct nfs_open_context {
 
 	struct list_head list;
 	struct nfs4_threshold	*mdsthreshold;
+	struct rcu_head	rcu_head;
 };
 
 struct nfs_open_dir_context {