smb: client: Fix NULL pointer dereference in cifs_debug_dirs_proc_show()

Reading /proc/fs/cifs/open_dirs may hit a NULL dereference when
tcon->cfids is NULL.

Add NULL check before accessing cfids to prevent the crash.

Reproduction:
- Mount CIFS share
- cat /proc/fs/cifs/open_dirs

Fixes: 844e5c0 ("smb3 client: add way to show directory leases for improved debugging")
Signed-off-by: Wang Zhaolong <wangzhaolong@huaweicloud.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Wang Zhaolong

fs/smb/client/cifs_debug.c

@@ -304,6 +304,8 @@ static int cifs_debug_dirs_proc_show(struct seq_file *m, void *v)
 			list_for_each(tmp1, &ses->tcon_list) {
 				tcon = list_entry(tmp1, struct cifs_tcon, tcon_list);
 				cfids = tcon->cfids;
+				if (!cfids)
+					continue;
 				spin_lock(&cfids->cfid_list_lock); /* check lock ordering */
 				seq_printf(m, "Num entries: %d\n", cfids->num_entries);
 				list_for_each_entry(cfid, &cfids->entries, entry) {
@@ -319,8 +321,6 @@ static int cifs_debug_dirs_proc_show(struct seq_file *m, void *v)
 					seq_printf(m, "\n");
 				}
 				spin_unlock(&cfids->cfid_list_lock);
-
-
 			}
 		}
 	}