vxlan: vxlan_core: Add support for FDB flush

The merge commit 92716869 ("Merge branch 'br-flush-filtering'")
added support for FDB flushing in bridge driver only, the VXLAN driver does
not support such flushing. Extend VXLAN driver to support FDB flushing.
In this commit, add support for flushing with state and flags, which are
the fields that supported in the bridge driver.

Note that bridge driver supports 'NTF_USE' flag, but there is no point to
support this flag for flushing as it is ignored when flags are stored.
'NTF_STICKY' is not relevant for VXLAN driver.

'NTF_ROUTER' is not supported in bridge driver for flush as it is not
relevant for bridge, add it for VXLAN.

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>