e2fsck: fix off-by-one bounds check on group number

e2fsck: fix off-by-one bounds check on group number

Since fs->group_desc_count is the number of block groups, the number
of the last group is always one less than this count.  Fix the bounds
check to reflect that.

This flaw shouldn't have any user-visible side effects, since the
block bitmap test based on last_grp later on can handle overbig block
numbers.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>