addons: address: fix _cache_update to use the right set cache api

Ticket: CM-13967
Reviewed By: julien, nikhil
Testing Done: tested failing config in the bug

This patch fixes a cache_update problem caught during mtu updates.
Cache updates were failing silently, leaving stale cache values.
For the below config, ifupdown2 was falsely reporting an mtu error,
because the cache had a stale mtu default value

$ifquery peerlink-3 peerlink-3.4094
auto peerlink-3
iface peerlink-3
        bond-slaves swp32s0 swp32s1
        bond-mode 802.3ad
        mtu 9202

auto peerlink-3.4094
iface peerlink-3.4094
        address 27.0.0.11/32
        mtu 9202

$ifreload -a
warning: peerlink-3.4094: vlan dev mtu 9202 is greater than lower realdev peerlink-3 mtu 1500

Before patch:
sequence of events:
- build cache with current system running mtu
- link set mtu 9202 on peerlink-3
- update cache for peerlink-3 to 9202 <---- cache update fails
- when processing peerlink-3.4094, query cache for lowerdev peerlink-3
  mtu: this returns 1500   <--- stale cache value
- print warning

After patch:
sequence of events:
- build cache with current system running mtu
- link set mtu 9202 on peerlink-3
- update cache for peerlink-3 to 9202 <---- cache updates to 9202
- when processing peerlink-3.4094, query cache for lowerdev peerlink-3
  mtu: this returns 9202
- success and proceed

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>

ifupdownaddons/iproute2.py

@@ -251,7 +251,7 @@ class iproute2(utilsBase):
     def _cache_update(self, attrlist, value):
         if ifupdownflags.flags.DRYRUN: return
         try:
-            linkCache.add_attr(attrlist, value)
+            linkCache.set_attr(attrlist, value)
         except:
             pass