tcp: move ir_mark initialization to tcp_openreq_init

ir_mark initialization is done for both TCP v4 and v6, move it in the
common tcp_openreq_init function.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>