From: Olaf Kirch <okir@suse.de>

The attached patch fixes a problem with the 32bit statfs call on NFS file
systems.  Some NFS servers return a value of -1 for the f_files and f_ffree.
The current code would think this is a 64bit value that cannot be converted
to 32bits.  Consequently, the system call would always fail.

The patch adds two special if() to detect a value of -1 for f_files and
f_ffree.