fix typo in struct field name

Stat.OwerflowPages changed to Stat.OverflowPages
Merge pull request #14 from bmatsuo/typo-fix

fix typo
fix typo in struct field name

Stat.OwerflowPages changed to Stat.OverflowPages
Merge pull request #15 from bmatsuo/errno

use a mix of Errno and syscall.Errno
Merge remote-tracking branch 'upstream/master'