NFS: Convert lookups of the lock context to RCU

Speed up lookups of an existing lock context by avoiding the inode->i_lock,
and using RCU instead.

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

include/linux/nfs_fs.h

@@ -62,6 +62,7 @@ struct nfs_lock_context {
 	struct nfs_open_context *open_context;
 	fl_owner_t lockowner;
 	atomic_t io_count;
+	struct rcu_head	rcu_head;
 };
 
 struct nfs4_state;