MD: fix a null dereference

rdev->mddev could be null in start time.

Reported-by: Ming Lei <ming.lei@redhat.com>
Fix: 5a85071c(md: use a separate bio_set for synchronous IO.)
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>