btrfs: test for valid bdev before kobj removal in btrfs_rm_device

commit 99994cd btrfs: dev delete should remove sysfs entry
added a btrfs_kobj_rm_device, which dereferences device->bdev...
right after we check whether device->bdev might be NULL.

I don't honestly know if it's possible to have a NULL device->bdev
here, but assuming that it is (given the test), we need to move
the kobject removal to be under that test.

(Coverity spotted this)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -1680,11 +1680,11 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path)
 	if (device->bdev == root->fs_info->fs_devices->latest_bdev)
 		root->fs_info->fs_devices->latest_bdev = next_device->bdev;
 
-	if (device->bdev)
+	if (device->bdev) {
 		device->fs_devices->open_devices--;
-
-	/* remove sysfs entry */
-	btrfs_kobj_rm_device(root->fs_info, device);
+		/* remove sysfs entry */
+		btrfs_kobj_rm_device(root->fs_info, device);
+	}
 
 	call_rcu(&device->rcu, free_device);