md: don't impose the MD_SB_DISKS limit on arrays without metadata.

These arrays, created with "mdadm --build" don't benefit from a limit.
The default will be used, which is '0' and is interpreted as "don't
impose a limit".

Reported-by:  <ian_bruce@mail.ru>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>