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 <[e-mail]>
Reviewed-by: Mike Christie <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/iscsi/iscsi_target_tmr.c

···
 	struct iscsi_tmr_req *tmr_req = cmd->tmr_req;
 	struct se_tmr_req *se_tmr = cmd->se_cmd.se_tmr_req;
 	struct iscsi_tm *hdr = (struct iscsi_tm *) buf;
-	u64 ret, ref_lun;
+	u64 ref_lun;
+	int ret;
 
 	pr_debug("Got TASK_REASSIGN TMR ITT: 0x%08x,"
 		" RefTaskTag: 0x%08x, ExpDataSN: 0x%08x, CID: %hu\n",