cifs: fix missed refcounting of ipc tcon

Fix missed refcounting of IPC tcon used for getting domain-based DFS
root referrals.  We want to keep it alive as long as mount is active
and can be refreshed.  For standalone DFS root referrals it wouldn't
be a problem as the client ends up having an IPC tcon for both mount
and cache.

Fixes: c88f7dcd ("cifs: support nested dfs links over reconnect")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -3426,6 +3426,7 @@ static int connect_dfs_root(struct mount_ctx *mnt_ctx, struct dfs_cache_tgt_list
 	 */
 	mount_put_conns(mnt_ctx);
 	mount_get_dfs_conns(mnt_ctx);
+	set_root_ses(mnt_ctx);
 
 	full_path = build_unc_path_to_root(ctx, cifs_sb, true);
 	if (IS_ERR(full_path))