dm bufio: update last_accessed when relinking a buffer

dm bufio: update last_accessed when relinking a buffer

commit eb76faf53b1ff7a77ce3f78cc98ad392ac70c2a0 upstream.

The 'last_accessed' member of the dm_buffer structure was only set when
the the buffer was created.  This led to each buffer being discarded
after dm_bufio_max_age time even if it was used recently.  In practice
this resulted in all thinp metadata being evicted soon after being read
-- this is particularly problematic for metadata intensive workloads
like multithreaded small random IO.

'last_accessed' is now updated each time the buffer is moved to the head
of the LRU list, so the buffer is now properly discarded if it was not
used in dm_bufio_max_age time.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>