btrfs: change BUG_ON to assertion in tree_move_down()

There's only one caller of tree_move_down() that does not pass level 0
so the assertion is better suited here.

Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2024-02-06 23:06:46 +01:00
commit 56f335e043