This is indeed because of an uninitialized kobject for blk_mq_ctx.
The blk_mq_ctx kobjects are initialized in blk_mq_sysfs_init(), but it
goes loop over hctx_for_each_ctx(), i.e. it initializes only for
online CPUs. Thus, when a CPU is hotplugged, the ctx for the newly
onlined CPU is registered without initialization.
This patch fixes the issue by initializing the all ctx kobjects
belonging to each queue.