bcache: Convert try_wait to wait_queue_head_t

We never waited on c->try_wait asynchronously, so just use the standard
primitives.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 10ce0c8..c1c4419 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -735,8 +735,8 @@
 	 * basically a lock for this that we can wait on asynchronously. The
 	 * btree_root() macro releases the lock when it returns.
 	 */
-	struct closure		*try_harder;
-	struct closure_waitlist	try_wait;
+	struct task_struct	*try_harder;
+	wait_queue_head_t	try_wait;
 	uint64_t		try_harder_start;
 
 	/*