net: mscc: ocelot: create a function that replaces an existing VCAP filter

net: mscc: ocelot: create a function that replaces an existing VCAP filter

VCAP (Versatile Content Aware Processor) is the TCAM-based engine behind
tc flower offload on ocelot, among other things. The ingress port mask
on which VCAP rules match is present as a bit field in the actual key of
the rule. This means that it is possible for a rule to be shared among
multiple source ports. When the rule is added one by one on each desired
port, that the ingress port mask of the key must be edited and rewritten
to hardware.

But the API in ocelot_vcap.c does not allow for this. For one thing,
ocelot_vcap_filter_add() and ocelot_vcap_filter_del() are not symmetric,
because ocelot_vcap_filter_add() works with a preallocated and
prepopulated filter and programs it to hardware, and
ocelot_vcap_filter_del() does both the job of removing the specified
filter from hardware, as well as kfreeing it. That is to say, the only
option of editing a filter in place, which is to delete it, modify the
structure and add it back, does not work because it results in
use-after-free.

This patch introduces ocelot_vcap_filter_replace, which trivially
reprograms a VCAP entry to hardware, at the exact same index at which it
existed before, without modifying any list or allocating any memory.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>