Fix offset error in linux_dirent_t

[XuShaohua]

In struct linux_dirent:

• offset of d_name is: 18
• offset of d_reclen is: 16

While in nc::linux_dirent_t:

• offset of d_name is: 24
• offset of d_reclen is: 16

Thus output of examples/dir.rs is invalid.

[XuShaohua]

Remove mem::offset_of!(), as it is available in rust 1.77.