[XSA-371] Linux: blkback driver may leak persistent grants

ISSUE DESCRIPTION

The fix for XSA-365 includes initialization of pointers such that
subsequent cleanup code wouldn't use uninitialized or stale values.
This initialization went too far and may under certain conditions also
overwrite pointers which are in need of cleaning up. The lack of
cleanup would result in leaking persistent grants. The leak in turn
would prevent fully cleaning up after a respective guest has died,
leaving around zombie domains.

IMPACT

A malicious or buggy frontend driver may be able to cause resource leaks
from the corresponding backend driver. This can result in a host-wide
Denial of Sevice (DoS).

VULNERABLE SYSTEMS

All Linux versions having the fix for XSA-365 applied are vulnerable.
XSA-365 was classified to affect versions back to at least 3.11.