dm mpath: remove annoying message of 'blk_get_request() returned -11'

It is very normal to see allocation failure, especially with blk-mq
request_queues, so it's unnecessary to report this error and annoy
people.

In practice this 'blk_get_request() returned -11' error gets logged
quite frequently when a blk-mq DM multipath device sees heavy IO.

This change is marked for stable@ because the annoying message in
question was included in stable@ commit 7083abbb.

Fixes: 7083abbb ("dm mpath: avoid that path removal can trigger an infinite loop")
Cc: stable@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>