ublk: move ublk_mark_io_ready() out of __ublk_fetch()

ublk: move ublk_mark_io_ready() out of __ublk_fetch()

ublk_batch_prep_io() calls __ublk_fetch() while holding io->lock
spinlock. When the last IO makes the device ready, ublk_mark_io_ready()
tries to acquire ub->cancel_mutex which can sleep, causing a
sleeping-while-atomic bug.

Fix by moving ublk_mark_io_ready() out of __ublk_fetch() and into the
callers (ublk_fetch and ublk_batch_prep_io) after the spinlock is
released.

Reported-by: Jens Axboe <axboe@kernel.dk>
Fixes: b256795b3606 ("ublk: handle UBLK_U_IO_PREP_IO_CMDS")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>