/*
* This routine initiates segment writes when LFS is consuming too many
* resources. Ideally the pageout daemon would be able to direct LFS
* more subtly.
* XXX We have one static count of locked buffers;
* XXX need to think more about the multiple filesystem case.
*
* Called and return with lfs_lock held.
* If fs != NULL, we hold the segment lock for fs.
*/