net/sched: act_police: more accurate MTU policing

commit 4ddc844e upstream.

in current Linux, MTU policing does not take into account that packets at
the TC ingress have the L2 header pulled. Thus, the same TC police action
(with the same value of tcfp_mtu) behaves differently for ingress/egress.
In addition, the full GSO size is compared to tcfp_mtu: as a consequence,
the policer drops GSO packets even when individual segments have the L2 +
L3 + L4 + payload length below the configured valued of tcfp_mtu.

Improve the accuracy of MTU policing as follows:
 - account for mac_len for non-GSO packets at TC ingress.
 - compare MTU threshold with the segmented size for GSO packets.
Also, add a kselftest that verifies the correct behavior.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[dcaratti: fix conflicts due to lack of the following commits:
 - commit 2ffe0395 ("net/sched: act_police: add support for
   packet-per-second policing")
 - commit afe231d3 ("selftests: forwarding: Add tc-police tests")
 - commit 53b61f29 ("selftests: forwarding: Add tc-police tests for
   packets per second")]
Link: https://lore.kernel.org/netdev/876d597a0ff55f6ba786f73c5a9fd9eb8d597a03.1644514748.git.dcaratti@redhat.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>