before returning failure if there is partial failure
fix detection of partial failure - GNTTABOP_map_grant_ref can actually return
zero for partial failure, so we need to always check all the entries
to detect it
previously, DIAGNOSTIC kernel triggered panic() for partial failure,
and non-DIAGNOSTIC kernel did not detect it at all, leading to Dom0 page
fault later; since the mapping failure can be triggered by malicious
DomU via bad grant reference, it's important to expect the calls
to fail, and handle it gracefully without crashing Dom0
part of fixes for XSA-362
d477b21b26