rbtree: add rb_nsearch helper

This helper is used to find cloest node that is greater than the key.

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

This removes the roof limit of disks that one sheep can manage.

The vnode creation algorithm isn't changed, but the calculation of vdisk number
is changed:

  we assign each vdisk 1G bytes, so, e.g, 160G will have 160 vdisks and 1T
  will have 1024 vdisks.

In this way, we only need to add/remove associated vdisks from vdisk ring by
plug/unplug physical disks.

vdisks are now managed by rbtree instead of a fixed array.

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

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
tests/func/057: fix false output due to changes of md vdisk management

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