xen/mmu: If the revector fails, don't attempt to revector anything else.

If the P2M revectoring would fail, we would try to continue on by
cleaning the PMD for L1 (PTE) page-tables. The xen_cleanhighmap
is greedy and erases the PMD on both boundaries. Since the P2M
array can share the PMD, we would wipe out part of the __ka
that is still used in the P2M tree to point to P2M leafs.

This fixes it by bypassing the revectoring and continuing on.
If the revector fails, a nice WARN is printed so we can still
troubleshoot this.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/mmu.c

@@ -1238,7 +1238,8 @@ static void __init xen_pagetable_setup_done(pgd_t *base)
 			memblock_free(__pa(xen_start_info->mfn_list), size);
 			/* And revector! Bye bye old array */
 			xen_start_info->mfn_list = new_mfn_list;
-		}
+		} else
+			goto skip;
 	}
 	/* At this stage, cleanup_highmap has already cleaned __ka space
 	 * from _brk_limit way up to the max_pfn_mapped (which is the end of
@@ -1259,6 +1260,7 @@ static void __init xen_pagetable_setup_done(pgd_t *base)
 	 * anything at this stage. */
 	xen_cleanhighmap(MODULES_VADDR, roundup(MODULES_VADDR, PUD_SIZE) - 1);
 #endif
+skip:
 #endif
 	xen_post_allocator_init();
 }