Only wake up bmap release thread when necessary.  This improves the iozone -a performance from 12m --> 10m.  The CPU usage drops from 200% to 140% from snapshots of top. Also did a mix of other tests.
Only wake up bmap release thread when necessary.  This improves the iozone -a performance
from 12m --> 10m.  The CPU usage drops from 200% to 140% from snapshots of top. Also did
a mix of other tests.

before:

zhihui@yuzu: ~/projects-yuzu/slash2/msctl$ cat abc
opstats.bmap-release-write           288
opstats.fsrq.handle.release          2268
opstats.release-wakeup               501002263

after:

zhihui@yuzu: ~/projects-yuzu/slash2/msctl$ ./msctl -S /var/lib/zhihui -p opstats | grep release
opstats.bmap-release-write           288
opstats.fsrq.handle.release          2268
opstats.release-wakeup               577

pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable,pscedu/slash2-stable