nilfs2: Remove an uninitialization warning in nilfs_btree_propagate_v()

`make CONFIG_NILFS2_FS=m M=fs/nilfs2/` will give the following warnings:

fs/nilfs2/btree.c: In function 'nilfs_btree_propagate':
fs/nilfs2/btree.c:1882: warning: 'maxlevel' may be used uninitialized in this function
fs/nilfs2/btree.c:1882: note: 'maxlevel' was declared here

Set maxlevel = 0 to fix it.
Signed-off-by: Li Hong <lihong.hi@gmail.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/btree.c

@@ -1879,7 +1879,7 @@ static int nilfs_btree_propagate_v(struct nilfs_btree *btree,
 				   struct nilfs_btree_path *path,
 				   int level, struct buffer_head *bh)
 {
-	int maxlevel, ret;
+	int maxlevel = 0, ret;
 	struct nilfs_btree_node *parent;
 	struct inode *dat = nilfs_bmap_get_dat(&btree->bt_bmap);
 	__u64 ptr;