Before this patch both addmdb and delmdb events were printed the same,
now we'll get a "Deleted" string in front when delmdb is received.
Before:
$ bridge mdb add dev br0 port eth3 grp 239.0.0.1
(monitor) dev br0 port eth3 grp 239.0.0.1 temp
$ bridge mdb del dev br0 port eth3 grp 239.0.0.1
(monitor) dev br0 port eth3 grp 239.0.0.1 temp
^^ No way to differentiate between both events.

After:
$ bridge mdb add dev br0 port eth3 grp 239.0.0.1
(monitor) dev br0 port eth3 grp 239.0.0.1 temp
$ bridge mdb del dev br0 port eth3 grp 239.0.0.1
(monitor) Deleted dev br0 port eth3 grp 239.0.0.1 temp
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>