fs: check FAT cluster size

fs: check FAT cluster size

The cluster size specifies how many sectors make up a cluster.  A
cluster size of zero makes no sense, as it would mean that the
cluster is made up of no sectors.  This will later lead into a
division by zero in sect_to_clust(), so better take care of that
early.

The MAX_CLUSTSIZE define can reduced using a define to make some
room in low-memory system.  Unfortunately if the code reads a
filesystem with a bigger cluster size it will overflow the buffer.

Signed-off-by: Patrick Wildt <patrick@blueri.se>