busybox

df: Use statvfs instead of non-standard statfs

Platforms differ on what their implementations of statfs include.
Importantly, FreeBSD's does not include a f_frsize member inside struct
statfs. However, statvfs is specified by POSIX and includes everything
we need, so we can just use that instead.

Signed-off-by: James Clarke <jrtc27@jrtc27.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>