MINOR: tinfo: replace the tgid with tgid_bit in tgroup_info

MINOR: tinfo: replace the tgid with tgid_bit in tgroup_info

Now that the tgid is accessible from the thread, it's pointless to have
it in the group, and it was only set but never used. However we'll soon
frequently need the mask corresponding to the group ID and the risk of
getting it wrong with the +1 or to shift 1 instead of 1UL is important,
so let's store the tgid_bit there.