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

include/sys/zfs_context.h

@@ -626,6 +626,7 @@ extern void delay(clock_t ticks);
 #define	defclsyspri	0
 
 #define	CPU_SEQID	((uintptr_t)pthread_self() & (max_ncpus - 1))
+#define	CPU_SEQID_UNSTABLE	CPU_SEQID
 
 #define	kcred		NULL
 #define	CRED()		NULL