Using a transaction in btrfs_search_slot is only useful when we are
searching to add or modify the tree. When the function is used for
searching, insert length and mod arguments are 0, there is no need to
use a transaction.

No functional changes, changing for consistency.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NMarcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>