udp: relax atomic operation on sk->sk_rmem_alloc

atomic_add_return() is more expensive than atomic_add()
and seems overkill in UDP rx fast path.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240328144032.1864988-3-edumazet@google.com


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