xen: xenbus: Use helper function IS_ERR_OR_NULL()

Use IS_ERR_OR_NULL() to detect an error pointer or a null pointer
open-coding to simplify the code.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230817014736.3094289-1-lizetao1@huawei.com


Signed-off-by: Juergen Gross <jgross@suse.com>