netconsole: take over skb pool lifecycle from netpoll

The fallback skb pool fronted by find_skb() is netconsole's only client:
every other netpoll goes through __netpoll_setup() / netpoll_send_skb()
without ever touching np->skb_pool.

Today __netpoll_setup() and __netpoll_cleanup() create and destroy the
pool for everyone, paying ~48 KB of pre-allocated skbs per netpoll
instance that only netconsole uses, what a waste!

Move the responsibility to netconsole. __netpoll_setup() did this under
the RTNL, but netconsole enables targets from enabled_store() /
alloc_param_target() without it, while the teardown path flushes the pool
(cancel_work_sync() + skb_queue_purge()) under the RTNL from
netconsole_process_cleanups_core(). Initialising the queue head and the
refill work on every enable would therefore race that flush.

They only need initialising once: after a flush the queue head is left
valid and empty and cancel_work_sync() leaves the work re-armable. Set
them up in alloc_and_init(), while the target is not yet reachable, and
let the enable paths only refill the pool via refill_skbs(), which
serialises with the flush through skb_pool.lock. See discussions in [1]

Link: https://lore.kernel.org/all/alDMvD5S7TZnoD_V@gmail.com/ [1]
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260710-netconsole_move_more-v3-3-6f63f76b28bc@debian.org
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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