Add membar_sync

Provides the missing full barrier variant to the membar primitive set.

While not used right now, this is probably going to change down the
road.

Name taken from Solaris, to follow the existing routines.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #13907

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

@@ -46,6 +46,7 @@
 
 #define	membar_consumer()		smp_rmb()
 #define	membar_producer()		smp_wmb()
+#define	membar_sync()			smp_mb()
 
 #define	physmem				zfs_totalram_pages