md: move purge_directory in init_path_space()

Or it will falsely remove .stale directory for newly added disk. For startup,
we don't purge directory.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
tests/functional: fix false alarm due to md changes

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