linux: Consolidate internal_statvfs

Remove the internal_statvfs64.c and open code the implementation
on internal_statvfs.c.  The alpha is now unrequired, the generic
implementation also handles it.

Also, remove unused includes on internal_statvfs.c, and remove
unused arguments on __internal_statvfs{64}.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>