net: optimize enqueue_to_backlog() for the fast path

net: optimize enqueue_to_backlog() for the fast path

Add likely() and unlikely() clauses for the common cases:

Device is running.
Queue is not full.
Queue is less than half capacity.

Add max_backlog parameter to skb_flow_limit() to avoid
a second READ_ONCE(net_hotdata.max_backlog).

skb_flow_limit() does not need the backlog_lock protection,
and can be called before we acquire the lock, for even better
resistance to attacks.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251024090517.3289181-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>