Linux 4.15 rc6

One last rc at the end of the year - and a Happy New Year to everybody!

This would have been a very quiet week, if it wasn't for the final x86
PTI stuff - and that shows in the diffstat too. About half the rc6
work is x86 updates. The timing for this isn't wonderful, but it all
looks nice and clean.

Outside of the x86 updates, it's misc driver updates (usb, networking,
rdma, sound), some perf tooling, and misc random stuff (core
networking, some irq fixes).

With all the x86 pti work coming in late in the rc like this, I'm by
now almost guaranteed to do an rc8 this release, not because there are
any known problems, but simply because of the timing of the patches.

Go forth and test,