CVE request: xen: x86-64: don't crash Xen upon direct pv guest access

[Eugene Teo <eugene () redhat com>]

handle_gdt_ldt_mapping_fault() is intended to deal with indirect 
accesses (i.e. those caused by descriptor loads) to the GDT/LDT mapping 
area only. While for 32-bit segment limits indeed prevent the function 
being entered for direct accesses (i.e. a #GP fault will be raised even 
before the address translation gets done, on 64-bit even user mode 
accesses would lead to control reaching the BUG_ON() at the beginning of 
that function.

http://lists.xensource.com/archives/html/xen-devel/2010-11/msg01650.html
https://bugzilla.redhat.com/show_bug.cgi?id=658155

Thanks, Eugene