Relax some unnecessary unsigned type changes in ext2fs.

While the changes in r245820 are in line with the ext2 spec,
the code derived from UFS can use negative values so it is
better to relax some types to keep them as they were, and
somewhat more similar to UFS. While here clean some casts.

Some of the original types are still wrong and will require
more work.

Discussed with:	bde
MFC after:	3 days
Revert revision 251648.  style(9) requires an empty line here.

Reported by:	mdf@