These two functions use netlink RTM_NEWNEIGH and RTM_DELNEIGH messages
to add and delete entries from a bridge's fdb. The bridge itself is
not referenced in the arguments to the functions, only the name of the
device that is attached to the bridge (since a device can only be
attached to one bridge at a time, and must be attached for this
function to make sense, the kernel easily infers which bridge's fdb is
being modified by looking at the device name/index).