vxlan: mdb: Adjust function arguments

Adjust the function's arguments and rename it to allow it to be reused
by future call sites that only have access to 'struct
vxlan_mdb_entry_key', but not to 'struct vxlan_mdb_config'.

No functional changes intended.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>