fs_mgr: add support for "sync" mount flag

Change-Id: I6ad74cfbf98d30320785e3691df30dabc5c54cb9
Merge "fs_mgr: add support for "sync" mount flag" into cm-10.2
Merge branch 'cm-10.2' of https://github.com/CyanogenMod/android_system_core into DJL10.2