New Sysfs options for cache_pressure, dirty_writeback, dirty_expire_interval

Big thanks to @temasek for sharing. and to  Boy Petersen for adding.

https://github.com/temasek/android_kernel_samsung_smdk4412/commit/3c7feedbb8a11b8146ba8db2eea2ceae3b0950c4
https://github.com/temasek/android_kernel_samsung_smdk4412/commit/5372ebd49ba69df9c2d6161e3e61c41b0b3b0321
https://github.com/temasek/android_kernel_samsung_smdk4412/commit/efe298c85fa8c46fcbdd1aff7a0998bab1ff369a

I made my own configs/fixes and tuning as always :)