initrd_start is defined in init/do_mounts_initrd.c, which is only
included in kernel if CONFIG_BLK_DEV_INITRD=y.
Signed-off-by: Laurent Fasnacht <l@libres.ch>
Cc: linux-mips@linux-mips.org
Cc: trivial@kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10198/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>