net: netpoll: flush skb pool during cleanup

The netpoll subsystem maintains a pool of 32 pre-allocated SKBs per
instance, but these SKBs are not freed when the netpoll user is brought
down. This leads to memory waste as these buffers remain allocated but
unused.

Add skb_pool_flush() to properly clean up these SKBs when netconsole is
terminated, improving memory efficiency.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20241114-skb_buffers_v2-v3-2-9be9f52a8b69@debian.org


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