ublk: handle UBLK_IO_FETCH_REQ earlier

ublk: handle UBLK_IO_FETCH_REQ earlier

Check for UBLK_IO_FETCH_REQ early in __ublk_ch_uring_cmd() and skip the
rest of the checks in this case. This allows removing the checks for
NULL io->task and UBLK_IO_FLAG_OWNED_BY_SRV unset in io->flags, which
are only allowed for FETCH.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250620151008.3976463-5-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>