zram: remove workqueue for freeing removed pending slot

zram: remove workqueue for freeing removed pending slot

Commit a0c516cbfc74 ("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 <[email protected]>
Cc: Nitin Gupta <[email protected]>
Tested-by: Sergey Senozhatsky <[email protected]>
Cc: Jerome Marchand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>