net: netlink: add NLM_F_BULK delete request modifier

Add a new delete request modifier called NLM_F_BULK which, when
supported, would cause the request to delete multiple objects. The flag
is a convenient way to signal that a multiple delete operation is
requested which can be gradually added to different delete requests. In
order to make sure older kernels will error out if the operation is not
supported instead of doing something unintended we have to break a
required condition when implementing support for this flag, f.e. for
neighbors we will omit the mandatory mac address attribute.
Initially it will be used to add flush with filtering support for bridge
fdbs, but it also opens the door to add similar support to others.

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>