netfilter: fix wrong byte order in nf_ct_seqadj_set internal information

In commit 41d73ec0

, sequence number adjustments were moved to a
separate file. Unfortunately, the sequence numbers that are stored
in the nf_ct_seqadj structure are expressed in host byte order. The
necessary ntohl call was removed when the call to adjust_tcp_sequence
was collapsed into nf_ct_seqadj_set. This broke the FTP NAT helper.
Fix it by adding back the byte order conversions.

Reported-by: Dawid Stawiarski <dawid.stawiarski@netart.pl>
Signed-off-by: Phil Oester <kernel@linuxace.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>