netfilter: xt_sctp: sctp chunk mapping doesn't work

When user tries to map all chunks given in argument, kernel
works on a copy of the chunkmap, but at the end it doesn't
check the copy, but the orginal one.

Signed-off-by: Qu Haoran <haoran.qu@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>