xfs: clear BAD_SUMMARY if unmounting an unhealthy filesystem

If we know the filesystem metadata isn't healthy during unmount, we want
to encourage the administrator to run xfs_repair right away.  We can't
do this if BAD_SUMMARY will cause an unclean log unmount to force
summary recalculation, so turn it off if the fs is bad.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>