target: Make spc_get_write_same_sectors return sector_t

We already expect TFO->get_blocks() to return sector_t for zero value case
when doing WRITE_SAME to the end of the backend device, so go ahead and return
sector_t from spc_get_write_same_sectors() to handle this case properly.

Also, update the single iblock_execute_write_same() caller of this code.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/include/target/target_core_backend.h b/include/target/target_core_backend.h
index 3393ab1..5079109 100644
--- a/include/target/target_core_backend.h
+++ b/include/target/target_core_backend.h
@@ -52,7 +52,7 @@
 
 sense_reason_t	spc_parse_cdb(struct se_cmd *cmd, unsigned int *size);
 sense_reason_t	spc_emulate_report_luns(struct se_cmd *cmd);
-int	spc_get_write_same_sectors(struct se_cmd *cmd);
+sector_t	spc_get_write_same_sectors(struct se_cmd *cmd);
 
 sense_reason_t	sbc_parse_cdb(struct se_cmd *cmd, struct sbc_ops *ops);
 u32	sbc_get_device_rev(struct se_device *dev);