Merge branch 'net-af_packet-allow-joining-a-fanout-when-link-is-down'

Gur Stavi says:

====================
net: af_packet: allow joining a fanout when link is down

PACKET socket can retain its fanout membership through link down and up
and leave a fanout while closed regardless of link state.
However, socket was forbidden from joining a fanout while it was not
RUNNING.

This scenario was identified while studying DPDK pmd_af_packet_drv.
Since sockets are only created during initialization, there is no reason
to fail the initialization if a single link is temporarily down.

This patch allows PACKET socket to join a fanout while not RUNNING.

Selftest psock_fanout is extended to test this "fanout while link down"
scenario.

Selftest psock_fanout is also extended to test fanout create/join by
socket that did not bind or specified a protocol, which carries an
implicit bind.

v3: https://lore.kernel.org/cover.1728555449.git.gur.stavi@huawei.com
v2: https://lore.kernel.org/cover.1728382839.git.gur.stavi@huawei.com
v1: https://lore.kernel.org/cover.1728303615.git.gur.stavi@huawei.com
====================

Link: https://patch.msgid.link/cover.1728802323.git.gur.stavi@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>