[iflib] Allow concurrent execution of admin tasks from different interfaces

Hi,

In current iflib implementation there is a single task queue group used
for all admin tasks, defined with:

iflib.c:560 TASKQGROUP_DEFINE(if_config_tqg, 1, 1);

This does not allow for concurrent processing of tasks. In systems with
a large number of interfaces this may cause one long running task
(e.g. related to FW update) to trigger timeouts of other tasks.
Changing that to:

TASKQGROUP_DEFINE(if_config_tqg, mp_cpu, 1);

works, but might be an overkill. I don't see a way to dynamically add
new threads/cpus to already defined taskqgroup though. I'll be grateful
for any suggestions how to approach this issue.

Thanks,
Krzysiek