libata-scsi: avoid repeated calculation of number of TRIM ranges

Currently libata statically allows only 1-block (512-byte) payload
for each TRIM command. Each payload can carry 64 TRIM ranges since
each range requires 8 bytes.

It is silly to keep doing the calculation (512 / 8) in different
places. Hence, define the new ATA_MAX_TRIM_RNUM for the result.

Signed-off-by: Tom Yan <tom.ty89@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index b0ca322..3dca0d1 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -2319,7 +2319,7 @@
 	 * with the unmap bit set.
 	 */
 	if (ata_id_has_trim(args->id)) {
-		put_unaligned_be64(65535 * 512 / 8, &rbuf[36]);
+		put_unaligned_be64(65535 * ATA_MAX_TRIM_RNUM, &rbuf[36]);
 		put_unaligned_be32(1, &rbuf[28]);
 	}
 
@@ -3311,8 +3311,8 @@
 
 	buf = page_address(sg_page(scsi_sglist(scmd)));
 
-	if (n_block <= 65535 * 512 / 8) {
-		size = ata_set_lba_range_entries(buf, 512, block, n_block);
+	if (n_block <= 65535 * ATA_MAX_TRIM_RNUM) {
+		size = ata_set_lba_range_entries(buf, ATA_MAX_TRIM_RNUM, block, n_block);
 	} else {
 		fp = 2;
 		goto invalid_fld;