net: dsa: mv88e6xxx: Verify after ATU Load ops

ATU Load operations could fail silently if there's not enough space
on the device to hold the new entry. When this happens, the symptom
depends on the unknown flood settings. If unknown multicast flood is
disabled, the multicast packets are dropped when the ATU table is
full. If unknown multicast flood is enabled, the multicast packets
will be flooded to all ports. Either way, IGMP snooping is broken
when the ATU Load operation fails silently.

Do a Read-After-Write verification after each fdb/mdb add operation
to make sure that the operation was really successful, and return
-ENOSPC otherwise.

Fixes: defb05b9 ("net: dsa: mv88e6xxx: Add support for fdb_add, fdb_del, and fdb_getnext")
Signed-off-by: Joseph Huang <Joseph.Huang@garmin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250306172306.3859214-1-Joseph.Huang@garmin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>