md: raid5-cache: Remove set but unused variable

Remove the variable offset in r5c_tree_index() to avoid a "set but not
used" compilation warning when compiling with W=1.
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/raid5-cache.c

@@ -195,9 +195,7 @@ struct r5l_log {
 static inline sector_t r5c_tree_index(struct r5conf *conf,
 				      sector_t sect)
 {
-	sector_t offset;
-
-	offset = sector_div(sect, conf->chunk_sectors);
+	sector_div(sect, conf->chunk_sectors);
 	return sect;
 }