generic vxlan: don't learn non-unicast L2 destinations

generic vxlan: don't learn non-unicast L2 destinations

This patch avoids learning non-unicast targets in the vxlan FDB. They
are non-unicast and thus should be sent to the broadcast-IPv6 instead of
a unicast address

Link: https://lore.kernel.org/netdev/[email protected]/
Link: https://github.com/freifunk-gluon/gluon/issues/3191
Signed-off-by: David Bauer <[email protected]>