unsigned  short is potentially too small to track blocks within
a group; today it is safe due to restrictions in e2fsprogs but
we have _lo / _hi bits for group blocks with the intent to go
up to 32 bits, so clean this up now.

There are many more places where we use unsigned/int/unsigned int
to contain a group block but this should at least fix all the
short types.

I added a few comments to the struct ext4_group_info definition
as well.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>