nfsd: don't allow setting maxblksize after svc created

It's harmless to set this after the server is created, but also
ineffective, since the value is only used at the time of
svc_create_pooled().  So fail the attempt, in keeping with the pattern
set by write_versions, write_{lease,grace}time and write_recoverydir.

(This could break userspace that tried to write to nfsd/max_block_size
between setting up sockets and starting the server.  However, such code
wouldn't have worked anyway, and I don't know of any examples--rpc.nfsd
in nfs-utils, probably the only user of the interface, doesn't do that.)
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfsctl.c

@@ -1190,7 +1190,7 @@ static ssize_t write_maxblksize(struct file *file, char *buf, size_t size)
 			bsize = NFSSVC_MAXBLKSIZE;
 		bsize &= ~(1024-1);
 		mutex_lock(&nfsd_mutex);
-		if (nfsd_serv && nfsd_serv->sv_nrthreads) {
+		if (nfsd_serv) {
 			mutex_unlock(&nfsd_mutex);
 			return -EBUSY;
 		}