btrfs: defrag: change BUG_ON to assertion in btrfs_defrag_leaves()

[ Upstream commit 51d4be54 ]

The BUG_ON verifies a condition that should be guaranteed by the correct
use of the path search (with keep_locks and lowest_level set), an
assertion is the suitable check.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>