powerpc/kexec: Fix up KEXEC_CONTROL_CODE_SIZE missed during conversion

Commit 163f6876 missed one, resulting in
the following compile error:

  AS      arch/powerpc/kernel/misc_32.o
arch/powerpc/kernel/misc_32.S: Assembler messages:
arch/powerpc/kernel/misc_32.S:902: Error: unsupported relocation against KEXEC_CONTROL_CODE_SIZE
make[2]: *** [arch/powerpc/kernel/misc_32.o] Error 1
make[1]: *** [arch/powerpc/kernel] Error 2
make: *** [vmlinux] Error 2

I grepped arch/ and found no further instances.
Signed-off-by: Paul Collins <paul@ondioline.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/misc_32.S

@@ -899,7 +899,7 @@ relocate_new_kernel:
 
 	/* set a new stack at the bottom of our page... */
 	/* (not really needed now) */
-	addi	r1, r4, KEXEC_CONTROL_CODE_SIZE - 8 /* for LR Save+Back Chain */
+	addi	r1, r4, KEXEC_CONTROL_PAGE_SIZE - 8 /* for LR Save+Back Chain */
 	stw	r0, 0(r1)
 
 	/* Do the copies */