zebra:fix a zebra crash issue caused by mac change

zebra:fix a zebra crash issue caused by mac change

When the MAC address of the neighbor changes, a possible crash issue may occur.

In the zebra_evpn_local_neigh_update function, the value of old_zmac (n->mac) will be updated to the new MAC address when the neighbor's MAC address changes.
The pointer to the memory that this pointer points to may be released in the zebra_evpn_local_neigh_deref_mac function. This will cause old_zmac to become a dangling pointer. Accessing this dangling pointer in the zebra_evpn_ip_inherit_dad_from_mac function below will cause the zebra process to crash.

Here is the backtrace:
(gdb) bt
0  0x00007fc12c5f1fbf in raise () from /lib/x86_64-linux-gnu/libpthread.so.0
1  0x00007fc12d52e19c in core_handler (signo=11, siginfo=0x7ffda1fd1570, context=<optimized out>) at lib/sigevent.c:262
2  <signal handler called>
3  zebra_evpn_ip_inherit_dad_from_mac (zvrf=<optimized out>, old_zmac=0x5579ac3ca520, new_zmac=0x5579aba82f80, nbr=0x5579abd65ec0) at zebra/ze
4  0x00005579aa8dbf6d in zebra_evpn_local_neigh_update (zevpn=0x5579abb81440, ifp=ifp@entry=0x5579ab8a1640, ip=ip@entry=0x7ffda1fd1b40, macadd
   local_inactive=local_inactive@entry=253, dp_static=false) at zebra/zebra_evpn_neigh.c:1729
5  0x00005579aa9190a9 in zebra_vxlan_handle_kernel_neigh_update (ifp=ifp@entry=0x5579ab8a1640, link_if=link_if@entry=0x5579abd14f90, ip=ip@ent
   is_ext=is_ext@entry=false, is_router=<optimized out>, local_inactive=false, dp_static=false) at zebra/zebra_vxlan.c:3791
6  0x00005579aa8b3048 in netlink_ipneigh_change (h=0x7ffda1fd1d50, len=<optimized out>, ns_id=<optimized out>) at zebra/rt_netlink.c:3649
7  0x00005579aa8ac667 in netlink_parse_info (filter=filter@entry=0x5579aa8ab630 <netlink_information_fetch>, nl=nl@entry=0x5579ab5861e8, zns=z
   startup=startup@entry=0) at zebra/kernel_netlink.c:965
8  0x00005579aa8ac8c8 in kernel_read (thread=<optimized out>) at zebra/kernel_netlink.c:402
9  0x00007fc12d53e60b in thread_call (thread=thread@entry=0x7ffda1fd9fd0) at lib/thread.c:1834
10 0x00007fc12d4fba78 in frr_run (master=0x5579ab3a1740) at lib/libfrr.c:1155
11 0x00005579aa89c6e3 in main (argc=11, argv=0x7ffda1fda3c8) at zebra/main.c:485
(gdb) f 3
3  zebra_evpn_ip_inherit_dad_from_mac (zvrf=<optimized out>, old_zmac=0x5579ac3ca520, new_zmac=0x5579aba82f80, nbr=0x5579abd65ec0) at zebra/ze
1230	zebra/zebra_evpn_neigh.c: No such file or directory.
(gdb) p *old_zmac
Cannot access memory at address 0x5579ac3ca520
(gdb)

To fix this issue, the ZEBRA_MAC_DUPLICATE flag should be retrieved before old_zmac is released and used in the zebra_evpn_ip_inherit_dad_from_mac function.

Signed-off-by: Jack.zhang <hanyu.zly@alibaba-inc.com>