nilfs2: Fix return in nilfs_mkdir

Return NULL instead of passing zero to ERR_PTR.
  Fixes smatch warning:
     - fs/nilfs2/namei.c:261 nilfs_mkdir() warn: passing zero to 'ERR_PTR'

Fixes: 88d5baf69082 ("Change inode_operations.mkdir to return struct dentry *")
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>