sched: don't call task_group() many times in set_task_rq()

It improves perfomance, especially if autogroup enabled.

The size of set_task_rq() was 0x180 and now it is 0xa0.

Change-Id: I6524ba2d77503d86040226eaee3f3419d2cb388a
Signed-off-by: Andrew Vagin <avagin@openvz.org>