x86: don't latch wrong (stale) GS base addresses

x86: don't latch wrong (stale) GS base addresses

load_segments() writes selector registers before doing any of the base
address updates. Any of these selector loads can cause a page fault in
case it references the LDT, and the LDT page accessed was only recently
installed. Therefore the call tree map_ldt_shadow_page() ->
guest_get_eff_kern_l1e() -> toggle_guest_mode() would in such a case
wrongly latch the outgoing vCPU's GS.base into the incoming vCPU's
recorded state.

Split page table toggling from GS handling - neither
guest_get_eff_kern_l1e() nor guest_io_okay() need more than the page
tables being the kernel ones for the memory access they want to do.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>