vrrp: simplify macvlan creation

Create the macvlan interface in one netlink command rather than three (creation
of the macvlan in netlink_link_add_vmac function, set of the mac address in the
netlink_link_setlladdr function, set macvlan mode in the netlink_link_setmode
function).

This simplification:
1. avoids potential issues if the firt netlink command passes but not the
   next ones
2. reduces number of netlink messages (light optimization)

Signed-off-by: Thibaut Collet <thibaut.collet@6wind.com>
Merge pull request #110 from 6WIND/master

vrrp: simplify macvlan creation