In a recent warning reported by scan-build, datasets were found to be
using a blocking call in a critical section.

datasets.c:187:12: warning: Call to blocking function 'fgets' inside of critical section [unix.BlockInCriticalSection]
  187 |     while (fgets(line, (int)sizeof(line), fp) != NULL) {
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
datasets.c:292:12: warning: Call to blocking function 'fgets' inside of critical section [unix.BlockInCriticalSection]
  292 |     while (fgets(line, (int)sizeof(line), fp) != NULL) {
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
datasets.c:368:12: warning: Call to blocking function 'fgets' inside of critical section [unix.BlockInCriticalSection]
  368 |     while (fgets(line, (int)sizeof(line), fp) != NULL) {
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
datasets.c:442:12: warning: Call to blocking function 'fgets' inside of critical section [unix.BlockInCriticalSection]
  442 |     while (fgets(line, (int)sizeof(line), fp) != NULL) {
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
datasets.c:512:12: warning: Call to blocking function 'fgets' inside of critical section [unix.BlockInCriticalSection]
  512 |     while (fgets(line, (int)sizeof(line), fp) != NULL) {
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
5 warnings generated.

Upon an analysis of the code, it was established that the call to all
the affected parts of the code is made by the fn DatasetGet which seems
to be only used by DetectDatasetSetup. A call to these Setup fns are
made by the rule loaders at the initialization of the detection engine
for a setup of expected keywords. Since this happens even before the
engine is started and there is just one thread active at the time, these
calls can be exempted from being marked critical section.

Dataset data is saved, accessed, looked up from the common THash API
when the actual traffic is matched against them.

Bug 7398