tcmu: Skip Data-Out blocks before gathering Data-In buffer for BIDI case

For the bidirectional case, the Data-Out buffer blocks will always at
the head of the tcmu_cmd's bitmap, and before gathering the Data-In
buffer, first of all it should skip the Data-Out ones, or the device
supporting BIDI commands won't work.

Fixed: 26418649 ("target/user: Introduce data_bitmap, replace
		data_length/data_head/data_tail")
Reported-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Tested-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Cc: stable@vger.kernel.org # 4.6+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>