Before the fix in some rare cases, radiusvlan could report a receive
of frames from another interface. This could lead to a wrong
authentication of a device connected to a port.

The problem is caused by a behaviour that if a socket is created it
listens on all interfaces until the bind function is called. It might
happen that later calls of recvfrom function will handle frames
received from other interfaces. To prevent that, radiusvlan flushes
all frames received before socket is bound to an interface.
The lost of frames is meaningless since all frames that radiusvlan
listens for are sent periodically.
Signed-off-by: Adam Wujek <dev_public@wujek.eu>