dm bufio: don't take the lock in dm_bufio_shrink_count

dm bufio: don't take the lock in dm_bufio_shrink_count

dm_bufio_shrink_count() is called from do_shrink_slab to find out how many
freeable objects are there. The reported value doesn't have to be precise,
so we don't need to take the dm-bufio lock.

Suggested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
(cherry picked from commit d12067f428c037b4575aaeb2be00847fc214c24a)

Orabug: 29964645
Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: Shuning Zhang <sunny.s.zhang@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>