Merge branch 'bridge-add-a-limit-on-learned-fdb-entries'

Johannes Nixdorf says:

====================
bridge: Add a limit on learned FDB entries

Introduce a limit on the amount of learned FDB entries on a bridge,
configured by netlink with a build time default on bridge creation in
the kernel config.

For backwards compatibility the kernel config default is disabling the
limit (0).

Without any limit a malicious actor may OOM a kernel by spamming packets
with changing MAC addresses on their bridge port, so allow the bridge
creator to limit the number of entries.

Currently the manual entries are identified by the bridge flags
BR_FDB_LOCAL or BR_FDB_ADDED_BY_USER, atomically bundled under the new
flag BR_FDB_DYNAMIC_LEARNED. This means the limit also applies to
entries created with BR_FDB_ADDED_BY_EXT_LEARN but none of BR_FDB_LOCAL
or BR_FDB_ADDED_BY_USER, e.g. ones added by SWITCHDEV_FDB_ADD_TO_BRIDGE.

Link to the corresponding iproute2 changes:
https://lore.kernel.org/r/20230919-fdb_limit-v4-1-b4d2dc4df30f@avm.de

v4: https://lore.kernel.org/r/20230919-fdb_limit-v4-0-39f0293807b8@avm.de/
v3: https://lore.kernel.org/r/20230905-fdb_limit-v3-0-7597cd500a82@avm.de/
v2: https://lore.kernel.org/netdev/20230619071444.14625-1-jnixdorf-oss@avm.de/
v1: https://lore.kernel.org/netdev/20230515085046.4457-1-jnixdorf-oss@avm.de/
====================

Link: https://lore.kernel.org/r/20231016-fdb_limit-v5-0-32cddff87758@avm.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>