ensure q_usage_counter is held over bio splits

[Christoph Hellwig <hch@lst.de>]

Hi Jens,

current blk_submit_bio can call into the bio splitting code without
q_usage_counter held, which can lead to inconsistent limits beeing
applied for drivers that change the limits at runtime.

The first patch in the series is a small comment and naming cleanup,
and the second one ensures we always hold q_usage_counter before
even entering blk_mq_submit_bio.

Diffstat:
 blk-core.c |   14 +++++++++-----
 blk-mq.c   |   59 ++++++++++++++++++++++++++++-------------------------------
 blk-mq.h   |    2 +-
 3 files changed, 38 insertions(+), 37 deletions(-)