x86/shadow: fix ref-counting error handling

x86/shadow: fix ref-counting error handling

The old-Linux handling in shadow_set_l4e() mistakenly ORed together the
results of sh_get_ref() and sh_pin(). As the latter failing is not a
correctness problem, simply ignore its return value.

In sh_set_toplevel_shadow() a failing sh_get_ref() must not be
accompanied by installing the entry, despite the domain being crashed.

This is XSA-250.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>