dm mpath: return DM_MAPIO_REQUEUE on blk-mq rq allocation failure

dm mpath: return DM_MAPIO_REQUEUE on blk-mq rq allocation failure

BugLink: http://bugs.launchpad.net/bugs/1774063
[ Upstream commit 050af08ffb1b62af69196d61c22a0755f9a3cdbd ]

blk-mq will rerun queue via RESTART or dispatch wake after one request
is completed, so not necessary to wait random time for requeuing, we
should trust blk-mq to do it.

More importantly, we need to return BLK_STS_RESOURCE to blk-mq so that
dequeuing from the I/O scheduler can be stopped, this results in
improved I/O merging.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>