btrfs: use explicit initializer for seq_elem

Using {} as initializer for struct seq_elem does not properly initialize
the list_head member, but it currently works because it gets set through
btrfs_get_tree_mod_seq if 'seq' is 0.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index f55721f..9de772e 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1206,7 +1206,7 @@
 	struct ulist *roots = NULL;
 	struct ulist_iterator uiter;
 	struct ulist_node *node;
-	struct seq_list elem = {};
+	struct seq_list elem = SEQ_LIST_INIT(elem);
 	int ret = 0;
 
 	tmp = ulist_alloc(GFP_NOFS);
@@ -1610,7 +1610,7 @@
 	struct ulist *roots = NULL;
 	struct ulist_node *ref_node = NULL;
 	struct ulist_node *root_node = NULL;
-	struct seq_list tree_mod_seq_elem = {};
+	struct seq_list tree_mod_seq_elem = SEQ_LIST_INIT(tree_mod_seq_elem);
 	struct ulist_iterator ref_uiter;
 	struct ulist_iterator root_uiter;