blk-wbt: move disable check into get_limit()

blk-wbt: move disable check into get_limit()

BugLink: https://bugs.launchpad.net/bugs/1810998
Check it in one place, instead of in multiple places.

Tested-by: Anchal Agarwal <anchalag@amazon.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(backported from commit ffa358dcaae1f2f00926484e712e06daa8953cb4)
[mfo: backport:
 - blk-wbt.c:
   - hunk 1: refresh lower context lines due to lack of commit
     782f569774d7 ("blk-wbt: throttle discards like background
     writes"), not required / introduces a new thing/behavior.]
   - hunk 2: s/rq_wait_inc_below(rqw/atomic_inc_below(&rqw->inflight/
   - hunk 3: s/rq_wait_inc_below(rqw/atomic_inc_below(&rqw->inflight/
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>