repart: do not fail when CopyBlocks= is used in the initrd

When running in the initrd --root= is automatically set to /sysroot or /sysusr
but then using CopyBlocks fails due to a security measure:

root@particle-caba-1e47:~# systemd-repart --dry-run=no /dev/vda
No machine ID set, using randomized partition UUIDs.
Automatic discovery of backing block devices not permitted in --root= mode, refusing.

Follow-up for 5c08da586f

src/repart/repart.c

@@ -202,6 +202,7 @@ static AppendMode arg_append_fstab = APPEND_NO;
 static char *arg_generate_fstab = NULL;
 static char *arg_generate_crypttab = NULL;
 static Set *arg_verity_settings = NULL;
+static bool arg_relax_copy_block_security = false;
 
 STATIC_DESTRUCTOR_REGISTER(arg_node, freep);
 STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
@@ -8737,6 +8738,9 @@ static int parse_argv(int argc, char *argv[], X509 **ret_certificate, EVP_PKEY *
                         r = parse_path_argument(optarg, /* suppress_root= */ false, &arg_image);
                         if (r < 0)
                                 return r;
+
+                        arg_relax_copy_block_security = false;
+
                         break;
 
                 case ARG_IMAGE_POLICY:
@@ -9157,6 +9161,8 @@ static int parse_argv(int argc, char *argv[], X509 **ret_certificate, EVP_PKEY *
                         arg_root = strdup("/sysusr");
                 if (!arg_root)
                         return log_oom();
+
+                arg_relax_copy_block_security = true;
         }
 
         if (argc > optind) {
@@ -9831,7 +9837,9 @@ static int run(int argc, char *argv[]) {
         r = context_open_copy_block_paths(
                         context,
                         loop_device ? loop_device->devno :         /* if --image= is specified, only allow partitions on the loopback device */
-                                      arg_root && !arg_image ? 0 : /* if --root= is specified, don't accept any block device */
+                                      /* if --root= is specified, don't accept any block device, unless it
+                                       * was set automatically because we are in the initrd  */
+                                      arg_root && !arg_image && !arg_relax_copy_block_security ? 0 :
                                       (dev_t) -1);                 /* if neither is specified, make no restrictions */
         if (r < 0)
                 return r;