net: make softnet_data.defer_count an atomic

This is preparation work to remove the softnet_data.defer_lock,
as it is contended on hosts with large number of cores.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250928084934.3266948-2-edumazet@google.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>