udp: add udp_drops_inc() helper

udp: add udp_drops_inc() helper

Generic sk_drops_inc() reads sk->sk_drop_counters.
We know the precise location for UDP sockets.

Move sk_drop_counters out of sock_read_rxtx
so that sock_write_rxtx starts at a cache line boundary.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250916160951.541279-9-edumazet@google.com
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>