ext4: don't count free clusters from a corrupt block group

A bg that's been flagged "corrupt" by definition has no free blocks,
so that the allocator won't be tempted to use the damaged bg.
Therefore, we shouldn't count the clusters in the damaged group when
calculating free counts.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>