Kernel prepatch 4.9-rc2

The second 4.9 prepatch is out for testing,and Linus is asking for people to test one feature in particular: "Myfavorite new feature that I called out in the rc1 announcement (thevirtually mapped stacks) is possibly implicated in some crashes that DaveJones has been trying to figure out, so if you want to be helpful and tryto see if you can give more data, please make sure to enableCONFIG_VMAP_STACK."