x86/PV: guest_get_eff_kern_l1e() may still need to switch page tables

x86/PV: guest_get_eff_kern_l1e() may still need to switch page tables

While indeed unnecessary for pv_ro_page_fault(), pv_map_ldt_shadow_page()
may run when guest user mode is active, and hence may need to switch to
the kernel page tables in order to retrieve an LDT page mapping.

Fixes: 9ff970564764 ("x86/mm: drop guest_get_eff_l1e()")
Reported-by: Manuel Bouyer <bouyer@antioche.eu.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Manuel Bouyer <bouyer@antioche.eu.org>