common/rc: Fix _supports_filetype function

generic/401 failed on RHEL6.8GA because "--output=xxx"
option is not supported by df.  So we remove it.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>