fstab-generator: fix ordering of /sysroot/usr mount

According to bootup(7) and the behavior when /usr is specified in /etc/fstab, the /sysroot/usr mount should be before initrd-fs.target, not before initrd-root-fs.target.

src/fstab-generator/fstab-generator.c

@@ -574,7 +574,7 @@ static int add_sysroot_usr_mount(void) {
                          false,
                          false,
                          false,
-                         SPECIAL_INITRD_ROOT_FS_TARGET,
+                         SPECIAL_INITRD_FS_TARGET,
                          "/proc/cmdline");
 }