virtio_net: split free_unused_bufs()

[ Upstream commit 6e345f8c7cd029ad3aaece15ad4425ac26e4eb63 ]

This patch separates two functions for freeing sq buf and rq buf from
free_unused_bufs().

When supporting the enable/disable tx/rq queue in the future, it is
necessary to support separate recovery of a sq buf or a rq buf.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220801063902.129329-40-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Stable-dep-of: f8bb51043945 ("virtio_net: suppress cpu stall when free_unused_bufs")
Signed-off-by: Sasha Levin <sashal@kernel.org>