LU-8976 libext2fs: fix maximum bg overhead calculation with meta_bg enabled

LU-8976 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.

E2fsprogs-commit: 1abdd04eb1d19488493022f81c03f5b80aa7fcc7
Change-Id: I22ef75037807f811b2133f4f4a62fbad02f1444f
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>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-on: https://review.whamcloud.com/25263
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>