dm writecache: pause writeback if cache full and origin being written directly

Implementation reuses dm_io_tracker, that until now was only used by
dm-cache, to track if any writes were issued directly to the origin
(due to cache being full) within the last second. If so writeback is
paused for a second.

This change improves performance for when the cache is full and IO is
issued directly to the origin device (rather than through the cache).

Depends-on: d53f1faf ("dm writecache: do direct write if the cache is full")
Suggested-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>