ZAP: Align za_name in zap_attribute_t to 8 bytes

Our code reading/writing there may not handle misaligned accesses
on a platforms that may care about it. I don't see a point to
complicate it to satisfy UBSan in CI. This alignment costs nothing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15921
Closes #16606