NFS: Fix put_nfs_open_context

We need to grab the inode->i_lock atomically with the last reference put in
order to remove the open context that is being freed from the
nfsi->open_files list.

Fix by converting the kref to a standard atomic counter and then using
atomic_dec_and_lock()...

Thanks to Arnd Bergmann for pointing out the problem.

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

include/linux/nfs_fs.h

@@ -71,7 +71,7 @@ struct nfs_access_entry {
 
 struct nfs4_state;
 struct nfs_open_context {
-	struct kref kref;
+	atomic_t count;
 	struct path path;
 	struct rpc_cred *cred;
 	struct nfs4_state *state;