blk-wbt: abstract out end IO completion handler

blk-wbt: abstract out end IO completion handler

BugLink: https://bugs.launchpad.net/bugs/1810998
Prep patch for calling the handler from a different context,
no functional changes in this patch.

Tested-by: Agarwal, Anchal <anchalag@amazon.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(backported from commit 061a5427530633de93ace4ef001b99961984af62)
[mfo: backport: __wbt_done():
 - keep signature (not static; parameters; no 'rqos')
 - remove the cast to 'rwb' from 'rqos' (it doesn't exist).]
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>