kboot: Disks should be at least 16MB

Linux pre-boot environments will often have a number of psuedo disks
that are small, all smaller than a few MB. 16MB is a good cutoff since
it's big enough to filter these devices, yet small enough to allow a
super-minimal partition through (the smallest I've been able to make
that's useful lately is around 20MB).

Sponsored by: Netflix

(cherry picked from commit 0386255bee079922f00f37eeb4e9e3138b7666c1)