RFR: 8229872: (fs) Increase buffer size used with getmntent

[Florian Weimer]

* Alan Bateman:

> Have you considered using fseek instead of rewind to avoid the
> uncertainty as to whether errno is set?

POSIX requires that rewind sets errno on failure, so I'm not sure if
this is necessary.  At least with rewind, you don't have to worry if you
need to call fseek or fseeko or fseeko64.

Thanks,
Florian