nfsd: eliminate unused nfs4_callback.cb_stat

The cb_stat member of struct nfs4_callback is unused
since commit ff7d9756 nfsd: use static memory for callback program and stats

Signed-off-by: Benny Halevy <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

include/linux/nfsd/state.h

···
 	u32                     cb_ident;
 	/* RPC client info */
 	atomic_t		cb_set;     /* successful CB_NULL call */
-	struct rpc_stat         cb_stat;
 	struct rpc_clnt *       cb_client;
 };