cifs: fix wrong unlock before return from cifs_tree_connect()

It should unlock 'tcon->tc_lock' before return from cifs_tree_connect().

Fixes: fe67bd563ec2 ("cifs: avoid use of global locks for high contention data")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>