scsi: libiscsi: Fix error count for active session

Fix an error count for active session if the total_cmds is invalid on the
function iscsi_session_setup().  Decrement the number of active sessions
before the funcion return.

Link: https://lore.kernel.org/r/EDBAAA0BBBA2AC4E9C8B6B81DEEE1D6916A28542@DGGEML525-MBS.china.huawei.com


Reviewed-by: Lee Duncan <lduncan@suuse.com>
Signed-off-by: Wu Bo <wubo40@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>