drbd: fix potential get_ldev/put_ldev refcount imbalance during attach

Race:

drbd_adm_attach()               | async drbd_md_endio()
                                |
device->ldev is still NULL.     |
                                |
drbd_md_read(                   |
 .endio = drbd_md_endio;        |
 submit;                        |
 ....                           |
 wait for done == 1;            |       done = 1;
);                              |       wake_up();
.. lot of other stuff,          |
.. includeing taking and        |
...giving up locks,             |
.. doing further IO,            |
.. stuff that takes "some time" |
                                | while in this context,
                                | this is the next statement.
                                | which means this context was scheduled
.. only then, finally,          | away for "some time".
device->ldev = nbc;             |
                                |       if (device->ldev)
                                |               put_ldev()

Unlikely, but possible. I was able to provoke it "reliably"
by adding an mdelay(500); after the wake_up().
Fixed by moving the if (!NULL) put_ldev() before done = 1;

Impact of the bug was that the resulting refcount imbalance
could lead to premature destruction of the object, potentially
causing a NULL pointer dereference during a subsequent detach.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>