BUG/MEDIUM: fd/threads: fix again incorrect thread selection in wakeup broadcast

BUG/MEDIUM: fd/threads: fix again incorrect thread selection in wakeup broadcast

Commit c1640f79f ("BUG/MEDIUM: fd/threads: fix incorrect thread selection
in wakeup broadcast") fixed an incorrect range being used to pick a thread
when broadcasting a wakeup for a foreign thread, but the selection was still
wrong as the number of threads and their mask was taken from the current
thread instead of the target thread. In addition, the code dealing with the
wakeup of a thread from the same group was still relying on MAX_THREADS
instead of tg->count.

This could theoretically cause random crashes with more than one thread
group though this was never encountered.

This needs to be backported to 2.7.