...because there could be superblock updates in the journal that haven't
yet been checkpointed. The online scan is perfectly capable of tallying
the free blocks on its own, so teach it do that and make a more accurate
report.
Reported-by: Elana Hashman <Elana.Hashman@twosigma.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>