Since _validate_tres_usage_limits_for_{assoc,qos} differed only

by assignment of a value to the out_tres_limit_array (the 3rd
argument to the _validate_tres_usage_limits_for_qos function),
I added function _validate_tres_usage_limits with an added
bool argument to check whether the assignment should be made.
_validate_tres_usage_limits_for_{assoc,qos} just calls/returns
_validate_tres_usage_limits with the new bool argument being set for
expected functionality:

out_tres_limit_set == true  same as _validate_tres_usage_limits_for_qos
out_tres_limit_set == false same as
_validate_tres_usage_limits_for_assoc

I went with the call through approach instead of changing the existing
called function names; not knowing if future developments would warrant
the functions diverging further, and so keeping differing function
names a necessity.