Use statvfs() instead of statfs() system call

Use statvfs() system call instead of statfs() to get filesystems
statistics with sar since:
1) statfs() has been deprecated by the LSB (useful only to get fs
type which is not needed here),
2) statvfs() better handles large file sizes.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>