udp: refine __udp_enqueue_schedule_skb() test

udp: refine __udp_enqueue_schedule_skb() test

Commit 5a465a0da13e ("udp: Fix multiple wraparounds
of sk->sk_rmem_alloc.") allowed to slightly overshoot
sk->sk_rmem_alloc, when many cpus are trying
to feed packets to a common UDP socket.

This patch, combined with the following one reduces
false sharing on the victim socket under DDOS.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250916160951.541279-6-edumazet@google.com
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>