virtio_pci: document why we defer kfree

commit a1eb03f546d651a8f39c7d0692b1f7f5b4e7e3cd upstream.

The reason we defer kfree until release function is because it's a
general rule for kobjects: kfree of the reference counter itself is only
legal in the release function.

Previous patch didn't make this clear, document this in code.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/virtio/virtio_pci.c
Simple merge