Use fsck.f2fs -a instead of -f for faster boot

and run fsck with -f on clean shutdown instead.

With -f, fsck.f2fs always performs a full scan of the /data
partition regardless of whether the partition is clean or not.
The full scan takes more than 2 seconds on volantis-userdebug
and delays the OS boot.

With -a, the command does almost nothing when the partition
is clean and finishes within 20-30ms on volantis-userdebug.
When the partition has an error or its check point has
CP_FSCK_FLAG (aka "need_fsck"), the command does exactly the
same full scan as -f to fix it.

Bug: 21853106
Change-Id: I126263caf34c0f5bb8f5e6794454d4e72526ce38