Bad error path in GNTTABOP_map_grant

ISSUE DESCRIPTION

Grant table operations are expected to return 0 for success, and a negative number for errors. Some misplaced brackets cause one error path to return 1 instead of a negative value.
The grant table code in Linux treats this condition as success, and proceeds with incorrectly initialised state.

IMPACT

A buggy or malicious guest can construct its grant table in such a way that, when a backend domain tries to map a grant, it hits the incorrect error path.
This will crash a Linux based dom0 or backend domain.

VULNERABLE SYSTEMS

Systems running any version of Xen with the XSA-295 fixes are vulnerable. Systems which have not yet taken the XSA-295 fixes are not vulnerable.
Systems running a Linux based dom0 or driver domain are vulnerable.
Systems running a FreeBSD or NetBSD based dom0 or driver domain are not impacted, as they both treat any nonzero value as a failure.
The vulnerability of other systems will depend on how they behave when getting an unexpected positive number from the GNTTABOP_map_grant hypercall.