netfilter: xt_TCPMSS: SYN packets are allowed to contain data

The TCPMSS target is dropping SYN packets where:
  1) There is data, or
  2) The data offset makes the TCP header larger than the packet.

Both of these result in an error level printk. This printk has been
removed.

This change avoids dropping SYN packets containing data. If there
is also no MSS option (as well as data), one will not be added
because of possible complications due to the increased packet size.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Patrick McHardy <kaber@trash.net>