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/os/freebsd/spl/sys/sysmacros.h

@@ -80,6 +80,7 @@ extern "C" {
 #define	kpreempt_disable() critical_enter()
 #define	kpreempt_enable() critical_exit()
 #define	CPU_SEQID curcpu
+#define	CPU_SEQID_UNSTABLE curcpu
 #define	is_system_labeled()		0
 /*
  * Convert a single byte to/from binary-coded decimal (BCD).

include/os/linux/spl/sys/sysmacros.h

@@ -76,6 +76,7 @@
 #define	max_ncpus			num_possible_cpus()
 #define	boot_ncpus			num_online_cpus()
 #define	CPU_SEQID			smp_processor_id()
+#define	CPU_SEQID_UNSTABLE		raw_smp_processor_id()
 #define	is_system_labeled()		0
 
 #ifndef RLIM64_INFINITY