drbd: fix potential kernel BUG (NULL deref)

BUG trace would look like:
 lc_find
 drbd_rs_complete_io
 got_OVResult
 drbd_asender

Could be triggered by explicit, or IO-error policy based,
detach during online-verify.

We may only dereference mdev->resync, if we first get_ldev(), as the
disk may break any time, causing mdev->resync to disappear once all
ldev references have been returned.
Already in flight online-verify requests or replies may still come in,
which we then need to ignore.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>