md: avoid invalid memory access for array sb->dev_roles

we need to gurantee 'desc_nr' valid before access array
of sb->dev_roles.

In addition, we should avoid .load_super always return '0'
when level is LEVEL_MULTIPATH, which is not expected.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1487373 ("Memory - illegal accesses")
Fixes: 6a5cb53a ("md: no longer compare spare disk superblock events in super_load")
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>