hammerhead: Optimize I/O scheduling

Inspired by bacon. We aren't suffering from slowdowns
during bootanim, anyway this is the right way to do it.

Change-Id: Icf3d9d6e52926012a4b7fdfdc1456a37ad782e71
Merge remote-tracking branch 'mahdi/kk-4.4.3' into kk-4.4.3