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