arm64: kexec_file: Fix sparse warning

Sparse gets cross about us returning 0 from image_load(), which has a
return type of 'void *':

>> arch/arm64/kernel/kexec_image.c:130:16: sparse: sparse: Using plain integer as NULL pointer

Return NULL instead, as we don't use the return value for anything if it
does not indicate an error.

Cc: Benjamin Gwin <bgwin@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 108aa503 ("arm64: kexec_file: try more regions if loading segments fails")
Link: https://lore.kernel.org/r/202011091736.T0zH8kaC-lkp@intel.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/kexec_image.c

@@ -127,7 +127,7 @@ static void *image_load(struct kimage *image,
 				kernel_segment->mem, kbuf.bufsz,
 				kernel_segment->memsz);
 
-	return 0;
+	return NULL;
 }
 
 #ifdef CONFIG_KEXEC_IMAGE_VERIFY_SIG