sched/psi: Fix psi_seq initialization

With the seqcount moved out of the group into a global psi_seq,
re-initializing the seqcount on group creation is causing seqcount
corruption.

Fixes: 570c8efd ("sched/psi: Optimize psi_group_change() cpu_clock() usage")
Reported-by: Chris Mason <clm@meta.com>
Suggested-by: Beata Michalska <beata.michalska@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>