libquota: quota file read support

libquota: quota file read support

This patch adds read quota file support, which includes:
- Improve scan dquot APIs & fix defects in scan dquot functions;
- Implement quota_file_open();
- Introduce quota_update_inode() to update usage in old quota file,
  and keep the limits unchanged.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>