dissect: Don't count RPMB and boot partitions

Filter-out RPMB partitions and boot partitions from MMC devices when
counting partitions enumerated by the kernel.

This complement the previous fixes to the problem reported in
systemd/systemd#5806

https://phabricator.endlessm.com/T21744

src/shared/dissect-image.c

@@ -277,8 +277,21 @@ int dissect_image(int fd, const void *root_hash, size_t root_hash_size, DissectI
                 /* Count the partitions enumerated by the kernel */
                 n = 0;
                 first = udev_enumerate_get_list_entry(e);
-                udev_list_entry_foreach(item, first)
+                udev_list_entry_foreach(item, first) {
+                        _cleanup_udev_device_unref_ struct udev_device *q;
+                        const char *sysname;
+
+                        /* Filter out MMC RPMB and Boot partitions, as they are not listed by blkid.
+                         * See https://github.com/systemd/systemd/issues/5806. */
+                        q = udev_device_new_from_syspath(udev, udev_list_entry_get_name(item));
+                        if (!q)
+                                return -errno;
+                        sysname = udev_device_get_sysname(q);
+                        if (sysname && startswith(sysname, "mmcblk") &&
+                                (endswith(sysname, "rpmb") || endswith(sysname, "boot0" ) || endswith(sysname, "boot1")))
+                                continue;
                         n++;
+                }
 
                 /* Count the partitions enumerated by blkid */
                 z = blkid_partlist_numof_partitions(pl);