Bump the kernel to 4.15.x

[niusmallnan]

Now 4.15.x is the current stable branch.
https://www.kernel.org/

[kingsd041]

The default kernel in rancheros-v1.3.0-rc1 has been upgraded to 4.15.9.

# uname -r
4.15.9-rancher

[ryansch]

The default kernel in 1.3.0 final is 4.9.80. We're still waiting on 4.15.x to fix the overlay fs bugs that cause tar to die.

[niusmallnan]

@ryansch
I tried to bump the kernel to 4.15.x, but zfs cannot work. #2299

I must roll back the kernel to 4.9.x.
I will try 4.14.x in next release, 4.14.x another LTS version and provides newer features.

[ryansch]

@niusmallnan That's new enough to fix our bug: moby/moby#19647

I've opened #2311 to track this.