btrfs: use correct type for workqueue flags

Through all the local wrappers to alloc_workqueue, __alloc_workqueue_key
takes an unsigned int.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
index e386c29..ec2ee47 100644
--- a/fs/btrfs/async-thread.h
+++ b/fs/btrfs/async-thread.h
@@ -66,7 +66,7 @@
 BTRFS_WORK_HELPER_PROTO(scrubnc_helper);
 
 struct btrfs_workqueue *btrfs_alloc_workqueue(const char *name,
-					      int flags,
+					      unsigned int flags,
 					      int max_active,
 					      int thresh);
 void btrfs_init_work(struct btrfs_work *work, btrfs_work_func_t helper,