more I/O context cleanup

Hi Jens,

this series dusts off a few more bits in the I/O context handling, and
makes a chunk of code only needed by the BFQ I/O scheduler optional.

Diffstat:
 block/Kconfig             |    3 
 block/Kconfig.iosched     |    1 
 block/blk-ioc.c           |  153 +++++++++++++++++++---------------------------
 block/blk.h               |    6 +
 include/linux/iocontext.h |    7 +-
 5 files changed, 80 insertions(+), 90 deletions(-)