net: arp: Fix a potential bug in arp_notify()

Summary:
- In arp_wait_setup() and arp_wait_cancel(), g_arp_waiters
  is protected by a critical section.
- However, I noticed that arp_notify() does not protect the
  g_arp_waiters that would cause memory corruption
- This commit fixes the issue.

Impact:
- None

Testing:
- Tested with spresense:rndis_smp, spresense:rndis

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

net/arp/arp_notify.c

@@ -188,6 +188,9 @@ int arp_wait(FAR struct arp_notify_s *notify, unsigned int timeout)
 void arp_notify(in_addr_t ipaddr)
 {
   FAR struct arp_notify_s *curr;
+  irqstate_t flags;
+
+  flags = enter_critical_section();
 
   /* Find an entry with the matching IP address in the list of waiters */
 
@@ -207,6 +210,8 @@ void arp_notify(in_addr_t ipaddr)
           break;
         }
     }
+
+  leave_critical_section(flags);
 }
 
 #endif /* CONFIG_NET_ARP_SEND */