tgtd: fix leak of task_in_scsi flag

When target_mgmt_request() function receives unknown or unsupported
task management function code or task was already completed for
ABORT_TASK request, iscsi_tm_execute() and iser_tm_exec() functions
exit without clearing task_in_scsi flag even though scsi command is
not in processing.

This is because the current implementation of target_mgmt_request()
function returns -EINVAL or -EEXIST for the above cases and
iscsi_tm_execute() and iser_tm_exec() do not handle these return
codes.

This fixes the issue by changing the return value of
target_mgmt_request() to MGMT_REQ_FAILED for such cases.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>