ARM: 8702/1: head-common.S: Clear lr before jumping to start_kernel()

If CONFIG_DEBUG_LOCK_ALLOC=y, the kernel log is spammed with a few
hundred identical messages:

    unwind: Unknown symbol address c0800300
    unwind: Index not found c0800300

c0800300 is the return address from the last subroutine call (to
__memzero()) in __mmap_switched().  Apparently having this address in
the link register confuses the unwinder.

To fix this, reset the link register to zero before jumping to
start_kernel().

Fixes: 9520b1a1 ("ARM: head-common.S: speed up startup code")
Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/head-common.S

@@ -114,6 +114,7 @@ __mmap_switched:
 	str	r8, [r2]			@ Save atags pointer
 	cmp	r3, #0
 	strne	r10, [r3]			@ Save control register values
+	mov	lr, #0
 	b	start_kernel
 ENDPROC(__mmap_switched)