mmc: core: fix SD card request queue refcount underflow during shutdown

When system shutdown, kernel shall call shutdown function of mmc to stop
its request queue and clean it up, during which the request queue's kobject
shall be put once. In normal cases, if the SD card is removed, the
mmc_blk_remove routine releases all the resources and kobjects related to
the disk and request queue by decreasing their kref counts to 0. But if the
SD card is removed after its shutdown function is called, below kref count
underflow error shall be thrown out because the kref count was decreased
once during request queue cleanup by the shutdown function in advance. This
change fixes it by skipping request queue cleanup in the mmc blk routine if
the queue has been marked as dead.

[  166.187211] refcount_t: underflow; use-after-free.
[  166.187277] ------------[ cut here ]------------
[  166.187321] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[  166.187542] Workqueue: events_freezable mmc_rescan
[  166.187558] task: ffffffe673b96680 task.stack: ffffff8008418000
[  166.187579] pc : refcount_sub_and_test+0x64/0x78
[  166.187593] lr : refcount_sub_and_test+0x64/0x78
[  166.187605] sp : ffffff800841ba20 pstate : 60c00145
[  166.188319] Call trace:
[  166.188331]  refcount_sub_and_test+0x64/0x78
[  166.188343]  refcount_dec_and_test+0x18/0x24
[  166.188355]  kobject_put+0x5c/0x74
[  166.188374]  blk_put_queue+0x1c/0x28
[  166.188388]  disk_release+0x70/0x90
[  166.188402]  device_release+0x38/0x90
[  166.188429]  kobject_cleanup+0xc4/0x1c0
[  166.188441]  kobject_put+0x68/0x74
[  166.188455]  put_disk+0x20/0x2c
[  166.188467]  mmc_blk_put+0x9c/0xdc
[  166.188480]  mmc_blk_remove_req+0x110/0x120
[  166.188493]  mmc_blk_remove+0x14c/0x22c
[  166.188505]  mmc_bus_remove+0x24/0x34
[  166.188517]  device_release_driver_internal+0x13c/0x1e0
[  166.188528]  device_release_driver+0x24/0x30
[  166.188540]  bus_remove_device+0xdc/0x120
[  166.188552]  device_del+0x1e0/0x284
[  166.188564]  mmc_remove_card+0x68/0x7c
[  166.188577]  mmc_sd_remove+0x24/0x48
[  166.188588]  mmc_sd_detect+0x120/0x1a4
[  166.188600]  mmc_rescan+0xf4/0x384
[  166.188613]  process_one_work+0x1c0/0x3d4
[  166.188625]  worker_thread+0x224/0x344
[  166.188637]  kthread+0x120/0x130
[  166.188649]  ret_from_fork+0x10/0x18.

Change-Id: I5b79ed6daecd2f1517a985f68c0ce9389d38bcf5
Signed-off-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>