tun-metadata: Fix memory leak in tun_metadata_table_mod().

tun-metadata: Fix memory leak in tun_metadata_table_mod().

In testcase "tunnel - Geneve metadata", valgrind reports a memory leak with
the following call stack.
    xcalloc (util.c:95)
    tun_metadata_alloc (tun-metadata.c:89)
    tun_metadata_table_mod (tun-metadata.c:151)
    handle_tlv_table_mod (ofproto.c:7782)
    handle_openflow__ (ofproto.c:7961)
    handle_openflow (ofproto.c:8023)
    ofconn_run (connmgr.c:1427)
    connmgr_run (connmgr.c:363)
    ofproto_run (ofproto.c:1813)
    bridge_run__ (bridge.c:2899)
    bridge_run (bridge.c:2956)
    main (ovs-vswitchd.c:111)

Reported-by: William Tu <u9012063@gmail.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>