mptcp: fix 32 bit DSN expansion

mptcp: fix 32 bit DSN expansion

The current implementation of 32 bit DSN expansion is buggy.
After the previous patch, we can simply reuse the newly
introduced helper to do the expansion safely.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/120
Fixes: 70105a1bbb51 ("mptcp: Implement MPTCP receive path")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>