mptcp: Fix bit-magic to find free index

Actually, ffs() uses integers to find for the next free index. Thus, it
fills the remaining bits of the 8-bit bitfield by zero's. So, we won't
find the correct index.

This patch adds a check, to see if we are going out-of-bounds.

Reported-by: Vinothkumar Vivekanandan <vvinoth@watchy.in>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
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>