RCs are in fact the time and place for all sorts of bug fixes, not just those which concern code added by that particular merge window. However, in this case it seems the kexec fix was deemed inappropriate due to its size, risk of introducing other bugs or simply because it did not or would not get enough testing before the release.
Originally Posted by TheBlackCat
Or at least that's my guess, since the LKML thread itself isn't particularly enlightening and might give the wrong impression. Have a look at the actual patch and Dave's 2nd reply after the merge request, where he says that approach might not be the best anyway.