ublk_drv: fix build warning with -Wmaybe-uninitialized and one sparse warning

ublk_drv: fix build warning with -Wmaybe-uninitialized and one sparse warning

After applying -Wmaybe-uninitialized manually, two build warnings are
triggered:

drivers/block/ublk_drv.c:940:11: warning: ‘io’ may be used uninitialized [-Wmaybe-uninitialized]
  940 |         io->flags &= ~UBLK_IO_FLAG_ACTIVE;

drivers/block/ublk_drv.c: In function ‘ublk_ctrl_uring_cmd’:
drivers/block/ublk_drv.c:1531:9: warning: ‘ret’ may be used uninitialized [-Wmaybe-uninitialized]

Fix the 1st one by removing 'io->flags &= ~UBLK_IO_FLAG_ACTIVE;' which
isn't needed since the function always return successfully after setting
this flag.

Fix the 2nd one by always initializing 'ret'.

Also fix another sparse warning of 'sparse: sparse: incorrect type in return
expression' by changing return type of ublk_setup_iod().

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220716095344.222674-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>