[4.4] Security fixes

I've backported fixes for I²C and media controller devices, dealing with the lifetime of related cdev and struct device instances and some similar race conditions. Fixing the lifetime issue for watchdog devices looks impractical for 4.4, as it depends on a big refactoring in 4.5.

All but one of these are already included in or queued for the later stable branches. You dropped the I²C lifetime fix for 4.9, but I hope my previous replies persuaded you that it is valid.

Ben.