cifs: Fix xid leak in cifs_ses_add_channel()

Before return, should free the xid, otherwise, the
xid will be leaked.

Fixes: d70e9fa5 ("cifs: try opening channels after mounting")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>