Fix inverse sleep logic in buf_daemon()

[mav]

Summary

Before commit 3cec5c77d617 buf_daemon() went to longer 1s sleep if numdirtybuffers <= lodirtybuffers. After that commit new condition !BIT_EMPTY(BUF_DOMAINS, &bdlodirty) is opposite -- true when one or more more domains is above lodirtybuffers. As result, on freshly booted system with no dirty buffers buf_daemon() wakes up 10 times per second and probably only 1 time per second when there is actual work to do.

Test Plan

With the patch I see buf_daemon() wake up rate on freshly booted system drops to one time per second.

[kib]

Might be worth asking pho to run stress2 with the patch, to make sure that there is no other bugs uncovered by this.

[pho]

I ran a mix of random stress2 tests for 24 hours, without observing any problems.