base: Fix a potential race in PollQueue::setupAsyncIO
base: Fix a potential race in PollQueue::setupAsyncIO

There is a potential race between enabling asynchronous IO and
selecting the target for the SIGIO signal. This changeset move the
F_SETOWN call to before the F_SETFL call that enables SIGIO
delivery. This ensures that signals are always sent to the correct
process.

vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,vovojh/gem5,vovojh/gem5,hoangt/tpzsimul.gem5,vovojh/gem5,hoangt/tpzsimul.gem5,vovojh/gem5,vovojh/gem5,vovojh/gem5,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,pombredanne/http-repo.gem5.org-gem5-,hoangt/tpzsimul.gem5