rseq: Reset slice extension when scheduled

rseq: Reset slice extension when scheduled

When a time slice extension was granted in the need_resched() check on exit
to user space, the task can still be scheduled out in one of the other
pending work items. When it gets scheduled back in, and need_resched() is
not set, then the stale grant would be preserved, which is just wrong.

RSEQ already keeps track of that and sets TIF_RSEQ, which invokes the
critical section and ID update mechanisms.

Utilize them and clear the user space slice control member of struct rseq
unconditionally within the existing user access sections. That's just an
unconditional store more in that path.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251215155709.131081527@linutronix.de