core: fix dm-verity auto-discovery in MountImageUnit()

The implementation of MountImageUnit()/systemctl mount-image was
changed to use a /proc/self/fd path as the source, but that causes
the dm-verity files autodiscovery to fail, as it looks for files
in the same directory as the image.

Use the original file path when setting up dm-verity.

src/shared/mount-util.c

@@ -789,6 +789,7 @@ static int mount_in_namespace(
         bool mount_slave_created = false, mount_slave_mounted = false,
                 mount_tmp_created = false, mount_tmp_mounted = false,
                 mount_outside_created = false, mount_outside_mounted = false;
+        _cleanup_free_ char *chased_src_path = NULL;
         struct stat st, self_mntns_st;
         pid_t child;
         int r;
@@ -826,9 +827,10 @@ static int mount_in_namespace(
         if (r < 0)
                 return log_debug_errno(r == -ENOENT ? SYNTHETIC_ERRNO(EOPNOTSUPP) : r, "Target does not allow propagation of mount points");
 
-        r = chase_symlinks(src, NULL, CHASE_TRAIL_SLASH, NULL, &chased_src_fd);
+        r = chase_symlinks(src, NULL, 0, &chased_src_path, &chased_src_fd);
         if (r < 0)
                 return log_debug_errno(r, "Failed to resolve source path of %s: %m", src);
+        log_debug("Chased source path of %s to %s", src, chased_src_path);
 
         if (fstat(chased_src_fd, &st) < 0)
                 return log_debug_errno(errno, "Failed to stat() resolved source path %s: %m", src);
@@ -873,7 +875,7 @@ static int mount_in_namespace(
         mount_tmp_created = true;
 
         if (is_image)
-                r = verity_dissect_and_mount(FORMAT_PROC_FD_PATH(chased_src_fd), mount_tmp, options, NULL, NULL, NULL, NULL);
+                r = verity_dissect_and_mount(chased_src_fd, chased_src_path, mount_tmp, options, NULL, NULL, NULL, NULL);
         else
                 r = mount_follow_verbose(LOG_DEBUG, FORMAT_PROC_FD_PATH(chased_src_fd), mount_tmp, NULL, MS_BIND, NULL);
         if (r < 0)