nfsd: fix potential UAF in nfsd4_cb_getattr_release

Once we drop the delegation reference, the fields embedded in it are no
longer safe to access. Do that last.

Fixes: c5967721 ("NFSD: handle GETATTR conflict with write delegation")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -3078,9 +3078,9 @@ nfsd4_cb_getattr_release(struct nfsd4_callback *cb)
 	struct nfs4_delegation *dp =
 			container_of(ncf, struct nfs4_delegation, dl_cb_fattr);
 
-	nfs4_put_stid(&dp->dl_stid);
 	clear_bit(CB_GETATTR_BUSY, &ncf->ncf_cb_flags);
 	wake_up_bit(&ncf->ncf_cb_flags, CB_GETATTR_BUSY);
+	nfs4_put_stid(&dp->dl_stid);
 }
 
 static const struct nfsd4_callback_ops nfsd4_cb_recall_any_ops = {