fsck_msdos: set limitation to reduce the time in extreme case

In some particular SD card, Most of the clusters are discrete.
It will take a long time to manage the runtime memory and cluster.
Set a max limitation here to limit the whole time, otherwise it will
take too much time and a bad user experience.

Change-Id: I3212f52e19c9b2c888120ada9f9ce46d7ef37d58
1 file changed