mm: remove compressed copy from zram in-memory

Swap subsystem does lazy swap slot free with expecting the page
would be swapped out again so we can avoid unnecessary write.

But the problem in in-memory swap(ex, zram) is that it consumes
memory space until vm_swap_full(ie, used half of all of swap device)
condition meet. It could be bad if we use multiple swap device,
small in-memory swap and big storage swap or in-memory swap alone.

This patch makes swap subsystem free swap slot as soon as swap-read
is completed and make the swapcache page dirty so the page should
be written out the swap device to reclaim it.
It means we never lose it.

I tested this patch with kernel compile workload.

1. before

compile time : 9882.42
zram max wasted space by fragmentation: 13471881 byte
memory space consumed by zram: 174227456 byte
the number of slot free notify: 206684

2. after

compile time : 9653.90
zram max wasted space by fragmentation: 11805932 byte
memory space consumed by zram: 154001408 byte
the number of slot free notify: 426972

Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>
Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>