Changed passing global variable arg to function find_instance_name(arg)
instead of passing arg[0] and arg[2] separately. And removed exception
in .luacheckrc for file /extra/dist/tarantoolctl.in.

This change only solves linter warning, nothing else.

Fixed #4929.

Reviewed-by: Leonid Vasiliev <lvasiliev@tarantool.org>
Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>