target/user: Add support for bidirectional commands

target/user: Add support for bidirectional commands

Enable TCMU to handle bidirectional SCSI commands. In such cases,
entries in iov[] cover both the Data-In and the Data-Out buffers. The
first iov_cnt entries correspond to the Data-Out buffer, while the
remaining iov_bidi_cnt entries correspond to the Data-In buffer.

Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Vangelis Koukis <vkoukis@arrikto.com>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Orabug: 25395066
(cherry picked from commit e4648b014e03baee45d5f5146c1219b19e4e5f2f)
Signed-off-by: Kyle Fortin <kyle.fortin@oracle.com>
Reviewed-by: Shan Hai <shan.hai@oracle.com>