mm/swapfile.c: sort swap entries before free

To reduce the lock contention of swap_info_struct->lock when freeing
swap entry.  The freed swap entries will be collected in a per-CPU
buffer firstly, and be really freed later in batch.  During the batch
freeing, if the consecutive swap entries in the per-CPU buffer belongs
to same swap device, the swap_info_struct->lock needs to be
acquired/released only once, so that the lock contention could be
reduced greatly.  But if there are multiple swap devices, it is possible
that the lock may be unnecessarily released/acquired because the swap
entries belong to the same swap device are non-consecutive in the
per-CPU buffer.

To solve the issue, the per-CPU buffer is sorted according to the swap
device before freeing the swap entries.

With the patch, the memory (some swapped out) free time reduced 11.6%
(from 2.65s to 2.35s) in the vm-scalability swap-w-rand test case with
16 processes.  The test is done on a Xeon E5 v3 system.  The swap device
used is a RAM simulated PMEM (persistent memory) device.  To test
swapping, the test case creates 16 processes, which allocate and write
to the anonymous pages until the RAM and part of the swap device is used
up, finally the memory (some swapped out) is freed before exit.

[akpm@linux-foundation.org: tweak comment]
Link: http://lkml.kernel.org/r/20170525005916.25249-1-ying.huang@intel.com
Signed-off-by: Huang Ying <ying.huang@intel.com>
Acked-by: Tim Chen <tim.c.chen@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>