dm: remove unnecessary current->bio_list check when submitting split bio

The depth-first splitting is introduced in commit 18a25da8 ("dm:
ensure bio submission follows a depth-first tree walk"), which is used
to fix the potential deadlock in case of the misordering handling of
bios caused by bio_list. There're two paths submitting split bios,
dm_wq_work() from worker thread and submit_bio() from application. Back
upon that time, dm_wq_work() thread calls __split_and_process_bio()
directly and thus will not trigger this issue since bio_list doesn't
exist here. So this issue will only be triggered from application
calling submit_bio(), and the fix has to check if current->bio_list is
non-NULL to distinguish this case.

However since commit 0c2915b8 ("dm: fix missing imposition of
queue_limits from dm_wq_work() thread"), dm_wq_work() thread calls
submit_bio_noacct() and thus also uses bio_list. Since then all entries
into __split_and_process_bio() are under protection of bio_list, and
thus the checking of current->bio_list when determinning if the
depth-first principle should be used, seems kind of nonsense. After all
the checking always succeeds now.

Fixes: 0c2915b8 ("dm: fix missing imposition of queue_limits from dm_wq_work() thread")
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>