fat: cannot compare bytes and sectors

The code multiples the FAT size in sectors by the sector size and then tries to
compare that to the number of sectors in the 'getsize' variable.  While fixing
this, also change the initial value of 'getsize' as the division of FATBUFSIZE
by the sector size gets us FATBUFBLOCKS.

Signed-off-by: Sergei Shtylyov <sshtylyov@mvista.com>