sched: Fix bug in SCHED_IDLE interaction with group scheduling

One of the isolation modifications for SCHED_IDLE is the
unitization of sleeper credit.  However the check for this
assumes that the sched_entity we're placing always belongs to a
task.

This is potentially not true with group scheduling and leaves
us rummaging randomly when we try to pull the policy.

Signed-off-by: Paul Turner <pjt@google.com>
Cc: peterz@infradead.org
LKML-Reference: <alpine.DEB.1.00.0907101649570.29914@kitami.corp.google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>