bcache: update comment in sysfs.c

We have struct cached_dev allocated by kzalloc in register_bcache(),
which initializes all the fields of cached_dev with 0s. And commit
ce4c3e19 ("bcache: Replace bch_read_string_list() by
__sysfs_match_string()") has remove the string "default".

Update the comment.
Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/sysfs.c

@@ -16,7 +16,7 @@
 #include <linux/sort.h>
 #include <linux/sched/clock.h>
 
-/* Default is -1; we skip past it for struct cached_dev's cache mode */
+/* Default is 0 ("writethrough") */
 static const char * const bch_cache_modes[] = {
 	"writethrough",
 	"writeback",
@@ -25,7 +25,7 @@ static const char * const bch_cache_modes[] = {
 	NULL
 };
 
-/* Default is -1; we skip past it for stop_when_cache_set_failed */
+/* Default is 0 ("auto") */
 static const char * const bch_stop_on_failure_modes[] = {
 	"auto",
 	"always",