kernel/sem: fix issue with expired timeouts on group operations

The loop was not tracking the correct next node in the list correctly.

However, it happened that the fix is way more involved than just fixing
that small issue, due to the way that semaphore group timeouts work.

Instead of handling timeouts one-by-one, we have to handle all timeouts
in a semaphore group as one. To do that, we use the fact that the
timeout of the real thread is always found first in the kernel's
timeout_q, and if it has expired, we do not even look at the timeouts of
the dummy threads.

Change-Id: Iadcfd06f33c6b335efa2592b2c01eeb5ca67afde
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>