migration: only check page size match if RAM postcopy is enabled

Postcopy may also be advised for dirty-bitmap migration only, in which
case the remote page size will not be available and we'll instead read
bogus data, blocking migration with a mismatch error if the VM uses
hugepages.

Fixes: 58110f0acb ("migration: split common postcopy out of ram postcopy")
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Message-Id: <20210204163522.13291-1-s.reiter@proxmox.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -3808,7 +3808,7 @@ static int ram_load_precopy(QEMUFile *f)
                         }
                     }
                     /* For postcopy we need to check hugepage sizes match */
-                    if (postcopy_advised &&
+                    if (postcopy_advised && migrate_postcopy_ram() &&
                         block->page_size != qemu_host_page_size) {
                         uint64_t remote_page_size = qemu_get_be64(f);
                         if (remote_page_size != block->page_size) {