sheep: use available size for virtual node calculation

f_bfree includes reserved space for filesystem.  On the other hand,
f_bavail is the actually available size and 'df' also uses this value.
f_bavail looks more appropriate for virutal node calculation.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>