btrfs: remove BUG_ON in btrfs_rm_dev_replace_free_srcdev()

That was only an extra check to tackle a few bugs around this area, now
its safe to remove it.  Replace it by an ASSERT.
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2016,19 +2016,20 @@ void btrfs_rm_dev_replace_free_srcdev(struct btrfs_fs_info *fs_info,
 	}
 
 	btrfs_close_bdev(srcdev);
-
 	call_rcu(&srcdev->rcu, free_device);
 
-	/*
-	 * unless fs_devices is seed fs, num_devices shouldn't go
-	 * zero
-	 */
-	BUG_ON(!fs_devices->num_devices && !fs_devices->seeding);
-
 	/* if this is no devs we rather delete the fs_devices */
 	if (!fs_devices->num_devices) {
 		struct btrfs_fs_devices *tmp_fs_devices;
 
+		/*
+		 * On a mounted FS, num_devices can't be zero unless it's a
+		 * seed. In case of a seed device being replaced, the replace
+		 * target added to the sprout FS, so there will be no more
+		 * device left under the seed FS.
+		 */
+		ASSERT(fs_devices->seeding);
+
 		tmp_fs_devices = fs_info->fs_devices;
 		while (tmp_fs_devices) {
 			if (tmp_fs_devices->seed == fs_devices) {