fstab: add discard and writeback

to avoid the low storage issues where the device may get very slow.
Its safe to mount with writeback and it should give a little IO performance.

Change-Id: I216593da6798df24702fba82f3abdccec24594be
Merge branch 'cm-10.1' of https://github.com/razrqcom-dev-team/android_device_motorola_msm8960-common into update
Merge pull request #7 from pcarenza/jb-mr1

fstab add discard and writeback