libext2fs: fix maximum bg overhead calculation with meta_bg enabled

libext2fs: fix maximum bg overhead calculation with meta_bg enabled

When meta_bg is enabled at mkfs time, we put at most one group
descriptor block in each blockgroup.  Unfortunately, the calculation
of max overhead per bg doesn't know this, so mkfs fails when it isn't
strictly necessary.  Fix it, since Dave reported that he couldn't
create a 500TB ext4 filesystem.

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