fs: jbd2: backport from aosp-common-3.4-kernel (johnnyslt)

Conflicts:
	fs/ext4/super.c
Add dynamic writeback feature from 3.1, fransicsofranco

Conflicts:
	arch/arm/boot/compressed/piggy.xzkern
	fs/block_dev.c
	fs/fs-writeback.c
	include/linux/writeback.h
clean up some derpage