scoutfs: don't leak btree block refs

Somewhere in the mists of time (around when we removed path tracking
which held refs to blocks?) walking blocks to migrate started leaking
btree block references.  It was providing a pointer so the walk gave it
the block it found but the caller was never dropping that ref.

It wasn't doing anything with the result of the walk so we just don't
provide a block pointer and the walk will drop the ref for us.  This
will stop leaking refs, effectively pinning the ring in memory.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/btree.c

@@ -1646,7 +1646,7 @@ int scoutfs_btree_write_dirty(struct super_block *sb)
 
 			ret = btree_walk(sb, root,
 					 BTW_DIRTY | BTW_NEXT | BTW_MIGRATE,
-					 walk_key, walk_len, 0, &bt,
+					 walk_key, walk_len, 0, NULL,
 					 iter_key, &iter_len);
 			if (ret < 0)
 				goto out;