mptcp: avoid data corruption on reinsert

When updating a partially acked data fragment, we
actually corrupt it. This is irrelevant till we send
data on a single subflow, as retransmitted data, if
any are discarded by the peer as duplicate, but it
will cause data corruption as soon as we will start
creating non backup subflows.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Tested-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>