btrfs: stop passing transaction parameter to log tree walk functions

It's unncessary to pass a transaction parameter since struct walk_control
already has a member that points to the transaction, so we can make the
functions access the structure.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>