btrfs: change BUG_ONs to assertions in btrfs_qgroup_trace_subtree()

The only caller do_walk_down() of btrfs_qgroup_trace_subtree() validates
the value of level and uses it several times before it's passed as an
argument. Same for root_eb that's called 'next' in the caller.

Change both BUG_ONs to assertions as this is to assure proper interface
use rather than real errors.

Signed-off-by: David Sterba <dsterba@suse.com>