mkfs.minix: increase maximum minix v2 and v3 file system sizes

mkfs.minix: increase maximum minix v2 and v3 file system sizes

mkfs.minix misbehaves when attempting to create a large v2 or v3
filesystem.  I finally traced it down to attempting to create too many
inodes so that the first zone is past 65535 blocks in.  This obviously
doesn't work as the on-disk superblock says this is a 16 bit integer.

I wrote a patch that catches this, clamps to the absolute v2/v3 limit
(like it already does for v1), and sets the blocks per inode to a more
reasonable ratio when exceeding half a gigabyte.  Having a half-gig
filesystem with most files being smaller than 3k isn't really reasonable.

I suppose if you don't want to adjust inode sizes automatically you could
take that part out, and it will just crab sooner.

Given the non-attention in the code, I suspect nobody ever had cause to
try such a big minix filesystem.  Well I have my reasons involving some
deeply embedded work where ext2 would place too much strain on the
hardware.

Reviewed-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Joshua Hudson <joshudson@gmail.com>