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

include/sys/zap.h

@@ -377,6 +377,7 @@ typedef struct {
 	uint64_t za_num_integers;
 	uint64_t za_first_integer;	/* no sign extension for <8byte ints */
 	uint32_t za_name_len;
+	uint32_t za_pad;	/* We want za_name aligned to uint64_t. */
 	char za_name[];
 } zap_attribute_t;