lightnvm: prevent race condition on pblk remove

When we trigger nvm target remove during device hot unplug, there is
a probability to hit a general protection fault. This is caused by use
of nvm_dev thay may be freed from another (hot unplug) thread
(in the nvm_unregister function).

Introduce lock in nvme_ioctl_dev_remove function to prevent this
situation.

Signed-off-by: Marcin Dziegielewski <[e-mail]>
Reviewed-by: Javier González <[e-mail]>
Signed-off-by: Matias Bjørling <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/lightnvm/core.c

···
 		return -EINVAL;
 	}
 
+	down_read(&nvm_lock);
 	list_for_each_entry(dev, &nvm_devices, devices) {
 		ret = nvm_remove_tgt(dev, &remove);
 		if (!ret)
 			break;
 	}
+	up_read(&nvm_lock);
 
 	return ret;
 }