Remove some Linux FS hardwired restrictions and assumptions about the
block size. I was only allowing 4K block size FS's, and assumed that the group descriptor map was in the second block (not true when the blocksize if 1K), and that the number of blocks in a descriptor as 8 times the block size. Also moved some sanity checks around, and cleaned up a couple of seeks. Now works on the baleep image, which is where we first noticed the problem.