mptcp: fix bad handling of 32 bit ack wrap-around

When receiving 32 bits DSS ack from the peer, the MPTCP need
to expand them to 64 bits value. The current code is buggy
WRT detecting 32 bits ack wrap-around: when the wrap-around
happens the current unsigned 32 bit ack value is lower than
the previous one.

Additionally check for possible reverse wrap and make the helper
visible, so that we could re-use it for the next patch.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/204


Fixes: cc9d2566 ("mptcp: update per unacked sequence on pkt reception")
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>