blk-mq: add file comments and update copyright notices

None of the blk-mq files have an explanatory comment at the top
for what that particular file does. Add that and add appropriate
copyright notices as well.

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/blk-mq-cpu.c
Simple merge

diff --cc block/blk-mq-cpumap.c
Simple merge

diff --cc block/blk-mq-tag.c
Simple merge

diff --cc block/blk-mq.c
Simple merge