ib_isert: Avoid duplicate iscsit_increment_maxcmdsn call

This patch avoids a duplicate iscsit_increment_maxcmdsn() call for
ISER_IB_RDMA_WRITE within isert_map_rdma() + isert_reg_rdma_frwr(),
which will already be occuring once during isert_put_datain() ->
iscsit_build_rsp_pdu() operation.

It also removes the local conn->stat_sn assignment + increment,
and changes the third parameter to iscsit_build_rsp_pdu() to
signal this should be done by iscsi_target_mode code.

Tested-by: Moussa Ba <moussaba@micron.com>
Cc: <stable@vger.kernel.org> # v3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>