sched/core: Add protection against null-pointer dereference

p->grp is being accessed outside of lock which can cause null-pointer
dereference. Fix this and also add rcu critical section around access
of this data structure.

CRs-fixed: 985379
Change-Id: Ic82de6ae2821845d704f0ec18046cc6a24f98e39
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>