NFSD: unregister shrinker when nfsd_init_net() fails

syzbot is reporting UAF read at register_shrinker_prepared() [1], for
commit 7746b32 ("NFSD: add shrinker to reap courtesy clients on
low memory condition") missed that nfsd4_leases_net_shutdown() from
nfsd_exit_net() is called only when nfsd_init_net() succeeded.
If nfsd_init_net() fails due to nfsd_reply_cache_init() failure,
register_shrinker() from nfsd4_init_leases_net() has to be undone
before nfsd_init_net() returns.

Link: https://syzkaller.appspot.com/bug?extid=ff796f04613b4c84ad89 [1]
Reported-by: syzbot <syzbot+ff796f04613b4c84ad89@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 7746b32 ("NFSD: add shrinker to reap courtesy clients on low memory condition")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfsctl.c

@@ -1458,12 +1458,14 @@ static __net_init int nfsd_init_net(struct net *net)
 		goto out_drc_error;
 	retval = nfsd_reply_cache_init(nn);
 	if (retval)
-		goto out_drc_error;
+		goto out_cache_error;
 	get_random_bytes(&nn->siphash_key, sizeof(nn->siphash_key));
 	seqlock_init(&nn->writeverf_lock);
 
 	return 0;
 
+out_cache_error:
+	nfsd4_leases_net_shutdown(nn);
 out_drc_error:
 	nfsd_idmap_shutdown(net);
 out_idmap_error: