dm bufio: do buffer cleanup from a workqueue

Until now, DM bufio's waiting for IO from reclaim context in its
shrinker has caused kswapd to block; which results in systemic IO
stalls and even deadlock, e.g.:
https://www.redhat.com/archives/dm-devel/2020-March/msg00025.html

Here is Dave Chinner's problem description that motivated this fix,
from: https://lore.kernel.org/linux-fsdevel/20190809215733.GZ7777@dread.disaster.area/



"Waiting for IO in kswapd reclaim context is considered harmful -
kswapd context shrinker reclaim should be as non-blocking as possible,
and any back-off to wait for IO to complete should be done by the high
level reclaim core once it's completed an entire reclaim scan cycle of
everything....

What follows from that, and is pertinent in this situation, is that if
you don't block kswapd, then other reclaim contexts are not going to
get stuck waiting for it regardless of the reclaim context they use."

Continued elsewhere:

"The only way to fix this problem once and for all is to stop using
the shrinker as a mechanism to issue and wait on IO. If you need
background writeback of dirty buffers, do it from a WQ_MEM_RECLAIM
workqueue that isn't directly in the memory reclaim path and so can
issue writeback and block safely from a GFP_KERNEL context. Kick the
workqueue from the shrinker context, but get rid of the IO submission
and waiting from the shrinker and all the GFP_NOFS memory reclaim
recursion problems go away."

As such, this commit moves buffer cleanup to a workqueue.

Suggested-by: Dave Chinner <dchinner@redhat.com>
Reported-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>