md: do not count journal as spare in GET_ARRAY_INFO

GET_ARRAY_INFO counts journal as spare (spare_disks), which is not
accurate. This patch fixes this.

Reported-by: Yi Zhang <yizhan@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>