vduse: Use proper spinlock for IRQ injection

The IRQ injection work used spin_lock_irq() to protect the
scheduling of the softirq, but spin_lock_bh() should be
used.

With spin_lock_irq(), we noticed delay of more than 6
seconds between the time a NAPI polling work is scheduled
and the time it is executed.

Fixes: c8a6153b ("vduse: Introduce VDUSE - vDPA Device in Userspace")
Cc: xieyongji@bytedance.com

Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Message-Id: <20230705114505.63274-1-maxime.coquelin@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>