vxlan: mdb: Factor out a helper for remote entry size calculation

Currently, netlink notifications are sent for individual remote entries
and not for the entire MDB entry itself.

Subsequent patches are going to add MDB get support which will require
the VXLAN driver to reply with an entire MDB entry.

Therefore, as a preparation, factor out a helper to calculate the size
of an individual remote entry. When determining the size of the reply
this helper will be invoked for each remote entry in the MDB entry.

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>