[syzbot] [afs?] general protection fault in afs_atcell_get_link

[Kohei Enju]

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    b35233e7bfa0 Merge tag 'for-6.14/dm-fixes-2' of git://git...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1232704c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=317038cbd53153e8
> dashboard link: https://syzkaller.appspot.com/bug?extid=76a6f18e3af82e84f264
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17d3fc78580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=169fb874580000

#syz test

diff --git a/fs/afs/dynroot.c b/fs/afs/dynroot.c
index 9732a1e17db3..3ea5e388ee16 100644
--- a/fs/afs/dynroot.c
+++ b/fs/afs/dynroot.c
@@ -213,6 +213,9 @@ static const char *afs_atcell_get_link(struct dentry *dentry, struct inode *inod
 	if (!dentry) {
 		/* We're in RCU-pathwalk. */
 		cell = rcu_dereference(net->ws_cell);
+		if (!cell)
+			return ERR_PTR(-ENOENT);
+
 		if (dotted)
 			name = cell->name - 1;
 		else