Btrfs: add ref_count and free function for btrfs_bio

1: ref_count is simple than current RBIO_HOLD_BBIO_MAP_BIT flag
   to keep btrfs_bio's memory in raid56 recovery implement.
2: free function for bbio will make code clean and flexible, plus
   forced data type checking in compile.

Changelog v1->v2:
 Rename following by David Sterba's suggestion:
 put_btrfs_bio() -> btrfs_put_bio()
 get_btrfs_bio() -> btrfs_get_bio()
 bbio->ref_count -> bbio->refs

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/reada.c b/fs/btrfs/reada.c
index 4d3d4e5..0e7beea 100644
--- a/fs/btrfs/reada.c
+++ b/fs/btrfs/reada.c
@@ -461,7 +461,7 @@
 	spin_unlock(&fs_info->reada_lock);
 	btrfs_dev_replace_unlock(&fs_info->dev_replace);
 
-	kfree(bbio);
+	btrfs_put_bbio(bbio);
 	return re;
 
 error:
@@ -486,7 +486,7 @@
 		kref_put(&zone->refcnt, reada_zone_release);
 		spin_unlock(&fs_info->reada_lock);
 	}
-	kfree(bbio);
+	btrfs_put_bbio(bbio);
 	kfree(re);
 	return re_exist;
 }