scsi: target: iscsi: Use int type to store negative value

Change the 'ret' variable in iscsit_tmr_task_reassign() from u64 to int,
as it needs to store either negative value or zero returned by
iscsit_find_cmd_for_recovery().

Storing the negative error codes in unsigned type, or performing equality
comparisons (e.g., ret == -2), doesn't cause an issue at runtime [1] but
can be confusing.  Additionally, assigning negative error codes to
unsigned type may trigger a GCC warning when the -Wsign-conversion flag
is enabled.

No effect on runtime.

Link: https://lore.kernel.org/all/x3wogjf6vgpkisdhg3abzrx7v7zktmdnfmqeih5kosszmagqfs@oh3qxrgzkikf/

 #1
Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>