Introduce CPU_SEQID_UNSTABLE

Current CPU_SEQID users don't care about possibly changing CPU ID, but
enclose it within kpreempt disable/enable in order to fend off warnings
from Linux's CONFIG_DEBUG_PREEMPT.

There is no need to do it. The expected way to get CPU ID while allowing
for migration is to use raw_smp_processor_id.

In order to make this future-proof this patch keeps CPU_SEQID as is and
introduces CPU_SEQID_UNSTABLE instead, to make it clear that consumers
explicitly want this behavior.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #11142

module/zfs/aggsum.c

@@ -167,9 +167,7 @@ aggsum_add(aggsum_t *as, int64_t delta)
 	struct aggsum_bucket *asb;
 	int64_t borrow;
 
-	kpreempt_disable();
-	asb = &as->as_buckets[CPU_SEQID % as->as_numbuckets];
-	kpreempt_enable();
+	asb = &as->as_buckets[CPU_SEQID_UNSTABLE % as->as_numbuckets];
 
 	/* Try fast path if we already borrowed enough before. */
 	mutex_enter(&asb->asc_lock);