BUG/MINOR: threads: produce correct global mask for tgroup > 1

BUG/MINOR: threads: produce correct global mask for tgroup > 1

In thread_resolve_group_mask(), if a global thread number is passed
and it belongs to a group greater than 1, an incorrect shift resulted
in shifting that ID again which made it appear nowhere or in a wrong
group possibly. The bug was introduced in 2.5 with commit 627def9e5
("MINOR: threads: add a new function to resolve config groups and
masks") though the groups only starts to be usable in 2.7, so there
is no impact for this bug, hence no backport is needed.