net: skb_queue_purge_reason() optimizations

1) Exit early if the list is empty.

2) splice the list into a local list,
   so that we block hard irqs only once.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231003181920.3280453-1-edumazet@google.com


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