netfilter: nf_iterate: fix incorrect RCU usage

As noticed by Eric, nf_iterate doesn't use RCU correctly by
accessing the prev pointer of a RCU protected list element when
a verdict of NF_REPEAT is issued.

Fix by jumping backwards to the hook invocation directly instead
of loading the previous list element before continuing the list
iteration.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>