BUG/MINOR: fd: used the update list from the fd's group instead of tgid

BUG/MINOR: fd: used the update list from the fd's group instead of tgid

In _fd_delete_orphan() we try to remove the FD from its update list
which is supposed to be the current thread group's. However the function
might be called from another group during stopping or under isolation,
so FD is not queued in the current group's update list but in its own
group's list. Let's retrieve the group from the FD instead of using
tgid.

This should have no impact on existing code since there is no code path
calling fd_delete() under thread isolation for now, and other cases are
blocked in fd_delete().

This must be backported to 2.7.