Fix the mkfs final-block-group-too-short bug.

Fix the mkfs final-block-group-too-short bug.

mkfs would fail for certain sizes resulting in a final block-group
that was too small to accommodate the minimum number of admin blocks.
There was already work-around code for when a decremented "numgroups"
was 1.  This change applies that work-around code for larger values of
numgroups, too.  Also, there was an off-by-one error in the guard test
that would allow a few too-small partition sizes to slip through and
provoke the error (see tests/t2000-mkfs.sh for two examples).
This change fixes that, too.