kexec/dt-ops.c: Fix check against 'fdt_add_subnode' return value

Vicenç reported (via [1]) that currently executing kexec with
'--dtb' option and passing a .dtb which doesn't have a '/chosen' node
leads to the following error:

  # kexec -d --dtb dtb_without_chosen_node.dtb --append 'cmdline' --load Image

    dtb_set_property: fdt_add_subnode failed: <valid offset/length>
    kexec: Set device tree bootargs failed.

This happens because currently we check the return value of
'fdt_add_subnode()' function call in 'dt-ops.c' incorrectly:

   result = fdt_add_subnode(new_dtb, nodeoffset, node);
   if (result) {
   	dbgprintf("%s: fdt_add_subnode failed: %s\n", _func__,
   		fdt_strerror(result));
   	goto on_error;
   }

As we can see in 'fdt_rw.c', a positive return value from
'fdt_add_subnode()' function doesn't indicate an error.

We can see that the Linux kernel (see 'drivers/firmware/efi/libstub/fdt.c'
for example) also checks the 'fdt_add_subnode()' function against negative
return values for errors. See an example below from 'update_fdt()' function in
'drivers/firmware/efi/libstub/fdt.c':

   node = fdt_add_subnode(fdt, 0, "chosen");
   if (node < 0) {
   	status = node;
   	<..snip..>
   	goto fdt_set_fail;
   }

This patch fixes the same in 'kexec-tools'.

[1]. http://lists.infradead.org/pipermail/kexec/2018-October/021746.html



Cc: Simon Horman <horms@verge.net.au>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reported-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>