nfsd: initialise nfsd_info.mutex early.

nfsd_info.mutex can be dereferenced by svc_pool_stats_start()
immediately after the new netns is created.  Currently this can
trigger an oops.

Move the initialisation earlier before it can possibly be dereferenced.

Fixes: 7b207ccd ("svc: don't hold reference for poolstats, only mutex.")
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Closes: https://lore.kernel.org/all/c2e9f6de-1ec4-4d3a-b18d-d5a6ec0814a0@linux.ibm.com/Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfsctl.c

@@ -2169,6 +2169,8 @@ static __net_init int nfsd_net_init(struct net *net)
 	nn->nfsd_svcstats.program = &nfsd_program;
 	nn->nfsd_versions = NULL;
 	nn->nfsd4_minorversions = NULL;
+	nn->nfsd_info.mutex = &nfsd_mutex;
+	nn->nfsd_serv = NULL;
 	nfsd4_init_leases_net(nn);
 	get_random_bytes(&nn->siphash_key, sizeof(nn->siphash_key));
 	seqlock_init(&nn->writeverf_lock);

fs/nfsd/nfssvc.c

@@ -672,7 +672,6 @@ int nfsd_create_serv(struct net *net)
 		return error;
 	}
 	spin_lock(&nfsd_notifier_lock);
-	nn->nfsd_info.mutex = &nfsd_mutex;
 	nn->nfsd_serv = serv;
 	spin_unlock(&nfsd_notifier_lock);