nfsd: fix wrong check in write_v4_end_grace()

BugLink: https://bugs.launchpad.net/bugs/1822271

commit dd838821 upstream.

Commit 62a063b8 "nfsd4: fix crash on writing v4_end_grace before
nfsd startup" is trying to fix a NULL dereference issue, but it
mistakenly checks if the nfsd server is started. So fix it.

Fixes: 62a063b8 "nfsd4: fix crash on writing v4_end_grace before nfsd startup"
Cc: stable@vger.kernel.org
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: Yihao Wu <wuyihao@linux.alibaba.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/nfsd/nfsctl.c

@@ -1106,7 +1106,7 @@ static ssize_t write_v4_end_grace(struct file *file, char *buf, size_t size)
 		case 'Y':
 		case 'y':
 		case '1':
-			if (nn->nfsd_serv)
+			if (!nn->nfsd_serv)
 				return -EBUSY;
 			nfsd4_end_grace(nn);
 			break;