[XSA-367] Linux: netback fails to honor grant mapping errors

ISSUE DESCRIPTION

XSA-362 tried to address issues here, but in the case of the netback
driver the changes were insufficient: It left the relevant function
invocation with, effectively, no error handling at all. As a result,
memory allocation failures there could still lead to frontend-induced
crashes of the backend.

IMPACT

A malicious or buggy networking frontend driver may be able to crash
the corresponding backend driver, potentially affecting the entire
domain running the backend driver. In a typical (non-disaggregated)
system that is a host-wide denial of service (DoS).

VULNERABLE SYSTEMS

Linux versions from at least 2.6.39 onwards are vulnerable, when run in
PV mode. Earlier versions differ significantly in behavior and may
therefore instead surface other issues under the same conditions. Linux
run in HVM / PVH modes is not vulnerable.