appletalk: aarp: zero-initialize aarp_entry to prevent heap info leak

aarp_alloc() allocates struct aarp_entry without zeroing it, but only
initializes refcnt and packet_queue.  When an unresolved AARP entry is
created, hwaddr[ETH_ALEN] is left uninitialized.

aarp_seq_show() later prints this field with %pM when users read
/proc/net/atalk/arp.  This can expose 6 bytes of stale heap data for
each unresolved entry.

Fix this by zero-initializing struct aarp_entry at allocation time.

Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Ao Wang <wangao@seu.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260529105017.81531-1-zhaoyz24@mails.tsinghua.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/appletalk/aarp.c

@@ -393,7 +393,7 @@ static void aarp_purge(void)
  */
 static struct aarp_entry *aarp_alloc(void)
 {
-	struct aarp_entry *a = kmalloc_obj(*a, GFP_ATOMIC);
+	struct aarp_entry *a = kzalloc_obj(*a, GFP_ATOMIC);
 	if (!a)
 		return NULL;