blk-mq: fix sparse warning on missed __percpu annotation

'struct blk_mq_ctx' is  __percpu, so add the annotation
and fix the sparse warning reported from Fengguang:

	[block:for-linus 2/3] block/blk-mq.h:75:16: sparse: incorrect
	type in initializer (different address spaces)

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>