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/freebsd/spl/sys/atomic.h

@@ -59,6 +59,7 @@ extern uint64_t atomic_cas_64(volatile uint64_t *target, uint64_t cmp,
 
 #define	membar_consumer()		atomic_thread_fence_acq()
 #define	membar_producer()		atomic_thread_fence_rel()
+#define	membar_sync()			atomic_thread_fence_seq_cst()
 
 static __inline uint32_t
 atomic_add_32_nv(volatile uint32_t *target, int32_t delta)

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