Properly pad struct tx_cpu to cache line

We already use ____cacheline_aligned in many places, so add one more
instead of seems arbitrary char tc_pad[8].

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15402

include/sys/txg_impl.h

@@ -73,8 +73,7 @@ struct tx_cpu {
 	kcondvar_t	tc_cv[TXG_SIZE];
 	uint64_t	tc_count[TXG_SIZE];	/* tx hold count on each txg */
 	list_t		tc_callbacks[TXG_SIZE]; /* commit cb list */
-	char		tc_pad[8];		/* pad to fill 3 cache lines */
-};
+} ____cacheline_aligned;
 
 /*
  * The tx_state structure maintains the state information about the different