buf: Fix the dirtybufthresh check

[markj]

Summary

dirtybufthresh is a watermark, slightly below the high watermark for
dirty buffers. When a delayed write is issued, the dirtying thread will
start flushing buffers if the dirtybufthresh watermark is reached. This
helps ensure that the high watermark is not reached, otherwise
performance will degrade as clustering and other optimizations are
disabled (see buf_dirty_count_severe()).

When the buffer cache was partitioned into "domains", the dirtybufthresh
threshold checks were not updated. Fix this.

[rlibby]

LGTM.

I guess this case is intended for when a single-ish writer is outpacing the buf daemon?

[kib]

In D28901#647089, @rlibby wrote:

I guess this case is intended for when a single-ish writer is outpacing the buf daemon?

Kind of, yes.

This case is problematic mostly not because we kept a lot of buffers for buffer daemon to flush, but because the daemon cannot flush it at all. We own the vnode lock, which prevents buffer daemon from initiating the writes for our dirty delayed-write buffers, so we must help it.

[mckusick]

It appears that this change should be MFC'ed to 12.