nfsd4: use a single struct file for delegations

When we converted to sharing struct filess between nfs4 opens I went too
far and also used the same mechanism for delegations.  But keeping
a reference to the struct file ensures it will outlast the lease, and
allows us to remove the lease with the same file as we added it.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/state.h

@@ -81,6 +81,7 @@ struct nfs4_delegation {
 	atomic_t		dl_count;       /* ref count */
 	struct nfs4_client	*dl_client;
 	struct nfs4_file	*dl_file;
+	struct file		*dl_vfs_file;
 	struct file_lock	*dl_flock;
 	u32			dl_type;
 	time_t			dl_time;