ZAP: Align za_name in zap_attribute_t to 8 bytes

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

Fixes:		openzfs#15921
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.

include/sys/zap.h

@@ -377,7 +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;
-	char za_name[];
+	char za_name[] __aligned(8);
 } zap_attribute_t;
 
 void zap_init(void);