Fix loops over group descriptors to prevent 2**32-1 block number overflows

Fix loops over group descriptors to prevent 2**32-1 block number overflows

For loops iterating over all group descriptors, consistently define
first_block and last_block in a way that they are inclusive of the
range, and do not overflow.

Previously on the last block group we did a test of <= first +
dec_blocks; this would actually wrap back to 0 for a total block count
of 2^32-1

Also add handling of last block group which may be smaller.

Signed-off-by: Eric Sandeen <esandeen@redhat.com>