zram: remove workqueue for freeing removed pending slot

zram: remove workqueue for freeing removed pending slot

a0c516c ("zram: don't grab mutex in zram_slot_free_noity") introduced free
request pending code to avoid scheduling by mutex under spinlock and it
was a mess which made code lenghty and increased overhead.

Now, we don't need zram->lock any more to free slot so this patch reverts
it and then, tb_lock should protect it.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>