MFC r200086:

MFC r200086:
Change 'load' balancing mode algorithm:
- Instead of measuring last request execution time for each drive and
choosing one with smallest time, use averaged number of requests, running
on each drive. This information is more accurate and timely. It allows to
distribute load between drives in more even and predictable way.
- For each drive track offset of the last submitted request. If new request
offset matches previous one or close for some drive, prefer that drive.
It allows to significantly speedup simultaneous sequential reads.

PR:             kern/113885

git-svn-id: svn://svn.freebsd.org/base/stable/8@200285 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f