zap_leaf: make l_hash[] variable length to silence UBSAN

When UBSAN is active and OpenZFS is a debug build, the l_hash assert at
the bottom of zap_open_leaf() causes UBSAN to complain.

This follows the example in 786641d to shut it up.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#15964

include/sys/zap_leaf.h

@@ -132,7 +132,7 @@ typedef struct zap_leaf_phys {
 	 * with the ZAP_LEAF_CHUNK() macro.
 	 */
 
-	uint16_t l_hash[1];
+	uint16_t l_hash[];
 } zap_leaf_phys_t;
 
 typedef union zap_leaf_chunk {