vxlan_core: move some fdb helpers to non-static

This patch moves some fdb helpers to non-static
for use in later patches. Ideally, all fdb code
could move into its own file vxlan_fdb.c.
This can be done as a subsequent patch and is out
of scope of this series.

Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>