mptcp: Fix Bug #33: Correctly set seq in mptcp_skb_trim_head

new_seq is actually alread in host-byte-order. So, we do not need the
htonl here. Doing the htonl results in a wrong sequence-number of this
segment and random crashes afterwards.

Fixes: Issue #33 (https://github.com/multipath-tcp/mptcp/issues/33)
Fixes: 6f7d8e5348d5d (Use sk_data_ready-callback)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>