Add dynamic page-writeback control v1

b3e0dc1 introduced dynamic page-writeback.
This patch adds a simple control interface
like the one for fsync control to enable or
disable the feature.

This feature could be enhanced since its very common
that a user wants speed while he is using his phone
and stability while he isn't.

echo 1 > /sys/class/misc/wirteback/writeback_enabled
(1 = enabled (default), 0 = disabled)
Signed-off-by: Blechd0se <alex.christ@hotmail.de>

Change-Id: I451c8aaafe2f7023e9d76e9b1a3b6efe7d7300cb