x86: drop unnecessary page table walking in compat r/o M2P handling

x86: drop unnecessary page table walking in compat r/o M2P handling

We have a global variable where the necessary L2 table is recorded; no
need to inspect L4 and L3 tables (and this way a few less places will
eventually need adjustment when we want to support 5-level page tables).
Also avoid setting up the L3 entry, as the address range never gets used
anyway (it'll be dropped altogether in a subsequent patch).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Hongyan Xia <hongyxia@amazon.com>
Reviewed-by: Wei Liu <wl@xen.org>