bcache: make is_discard_enabled() static

It's not used outside this file.

Fixes: 63120731 ("bcache: fix failure in journal relplay")
Signed-off-by: NJens Axboe <axboe@kernel.dk>

drivers/md/bcache/journal.c

@@ -318,7 +318,7 @@ void bch_journal_mark(struct cache_set *c, struct list_head *list)
 	}
 }
 
-bool is_discard_enabled(struct cache_set *s)
+static bool is_discard_enabled(struct cache_set *s)
 {
 	struct cache *ca;
 	unsigned int i;