avoid a few q_usage_counter roundtrips v2

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens,

the way we track reference on q_usage_counter is a little weird at the
moment, in that we often have to grab another reference in addition to
the current one.  This small series reshuffles that to avoid the extra
references in the normal I/O path.

Changes since v1:
 - rebased to the lastest for-5.8/block tree with the blk-crypt addition