netfilter: defrag: remove one redundant atomic ops

Instead of doing one atomic operation per frag, we can factorize them.
Reported from Eric Dumazet.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>