cam iosched: Allow read bias to be 0

[imp]

Summary

When read bias is 0, we queue all requests to the same queue. When it's
non-zero, we separate the queues we use so we can bias reads vs writes
for workloads that are read centric. Prior to this change, when read
bias was 0, writes would be unfairly prioritized. These changes restore
the typical bias you get from disksort or ordered insertion at the end
of the list.

Sponsored by: Netflix

[chs]

I would characterize this change as: redefine "read_bias==0" from the current meaning of "prefer writes" to instead mean "no read-vs-write bias".
This patch looks correct in implementing that new meaning, though it would be nice if it didn't involve essentially duplicating the code of cam_iosched_get_write() in cam_iosched_next_bio().

[imp]

closed with cc1572ddeb8cd82879ce0cca634bf6a8830c0f40 with updated wording from @chs but didn't close automatically since i didn't notice git arc stage didn't put a review in.