zebra: Fix for heap-use-after-free in EVPN

zebra: Fix for heap-use-after-free in EVPN

Issue:
When a netns is deleted, since zebra doesn’t receive interface down/delete
notifications from kernel, it manually deletes the interface without removing
the association between zebra_l3vni and the interface that is being deleted
(i.e it deletes the interface without setting “zl3vni->vxlan_if” to NULL).

Later, during the deletion of netns, when zl3vni_rmac_uninstall() is called to
uninstall the remote RMAC from the kernel, zebra ends up accessing stale
“zl3vni->vxlan_if” pointer, which now points to freed memory.
This was causing heap use-after-free.

Fix:
Before zebra starts deleting the interfaces when it receives netns delete notification,
appropriate functions() are being called to remove the association between evpn structs
and interface and set “zl3vni->vxlan_if” to NULL. This ensures that when
zl3vni_rmac_uninstall() is called during netns deletion, it will bail because
“zl3vni->vxlan_if” is NULL.

Signed-off-by: Pooja Jagadeesh Doijode <pdoijode@nvidia.com>