net: dsa: allow matchall mirroring rules towards the CPU

If the CPU bandwidth capacity permits, it may be useful to mirror the
entire ingress of a user port to software.

This is in fact possible to express even if there is no net_device
representation for the CPU port. In fact, that approach was already
exhausted and that representation wouldn't have even helped [1].

The idea behind implementing this is that currently, we refuse to
offload any mirroring towards a non-DSA target net_device. But if we
acknowledge the fact that to reach any foreign net_device, the switch
must send the packet to the CPU anyway, then we can simply offload just
that part, and let the software do the rest. There is only one condition
we need to uphold: the filter needs to be present in the software data
path as well (no skip_sw).

There are 2 actions to consider: FLOW_ACTION_MIRRED (redirect to egress
of target interface) and FLOW_ACTION_MIRRED_INGRESS (redirect to ingress
of target interface). We don't have the ability/API to offload
FLOW_ACTION_MIRRED_INGRESS when the target port is also a DSA user port,
but we could also permit that through mirred to the CPU + software.

Example:

$ ip link add dummy0 type dummy; ip link set dummy0 up
$ tc qdisc add dev swp0 clsact
$ tc filter add dev swp0 ingress matchall action mirred ingress mirror dev dummy0

Any DSA driver with a ds->ops->port_mirror_add() implementation can now
make use of this with no additional change.

[1] https://lore.kernel.org/netdev/20191002233750.13566-1-olteanv@gmail.com/
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241023135251.1752488-6-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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