sheep: fix coredump of sheep daemon in disk mode

In disk mode, we calculate number of md vnode by using "nr = disk_space / WEIGHT_MIN",
when disk_space is smaller than WEIGHT_MIN (4G currently), the 'nr' will be zero.

Imaging a sheep daemon who has two data paths. One data path has 8GB space, but
another data path has only 1GB space. The nr_disk is 2 and all the vnode in md.vroot
is for the 8GB data path. Then the larger data path get a disk fail and be removed
from md hashing ring, now the md.vroot has no vnode but nr_disk is 1. When the code
routine comes to md_get_object_dir_nolock(), it will call oid_to_vdisk() and get NULL
result which will make sheep daemon coredump.
To fix this problem, we should calculate number of vnode by DIV_ROUND_UP in disk mode.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
test: add test case for disk fail when sheep has different size of paths

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>