nfsd: fix memory leak in nfsd_create_serv error paths

When nfsd_create_serv() calls percpu_ref_init() to initialize
nn->nfsd_net_ref, it allocates both a percpu reference counter
and a percpu_ref_data structure (64 bytes). However, if the
function fails later due to svc_create_pooled() returning NULL
or svc_bind() returning an error, these allocations are not
cleaned up, resulting in a memory leak.

The leak manifests as:
- Unreferenced percpu allocation (8 bytes per CPU)
- Unreferenced percpu_ref_data structure (64 bytes)

Fix this by adding percpu_ref_exit() calls in both error paths
to properly clean up the percpu_ref_init() allocations.

This patch fixes the percpu_ref leak in nfsd_create_serv() seen
as an auxiliary leak in syzbot report 099461f8558eb0a1f4f3; the
prepare_creds() and vsock-related leaks in the same report
remain to be addressed separately.

Reported-by: [e-mail]
Link: https://syzkaller.appspot.com/bug?extid=099461f8558eb0a1f4f3
Fixes: 47e988147f40 ("nfsd: add nfsd_serv_try_get and nfsd_serv_put")
Signed-off-by: Shardul Bankar <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfssvc.c

···
 	serv = svc_create_pooled(nfsd_programs, ARRAY_SIZE(nfsd_programs),
 				 &nn->nfsd_svcstats,
 				 nfsd_max_blksize, nfsd);
-	if (serv == NULL)
+	if (serv == NULL) {
+		percpu_ref_exit(&nn->nfsd_net_ref);
 		return -ENOMEM;
+	}
 
 	error = svc_bind(serv, net);
 	if (error < 0) {
 		svc_destroy(&serv);
+		percpu_ref_exit(&nn->nfsd_net_ref);
 		return error;
 	}
 	spin_lock(&nfsd_notifier_lock);