vxlan: vxlan_core: Make vxlan_flush() more generic for future use

The function vxlan_flush() gets a boolean called 'do_all' and in case
that it is false, it does not flush entries with state 'NUD_PERMANENT'
or 'NUD_NOARP'. The following patches will add support for FDB flush
with parameters from user space. Make the function more generic, so it
can be used later.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>