9p/trans_xen: replace simple_strto* with kstrtouint

9p/trans_xen: replace simple_strto* with kstrtouint

In xen_9pfs_front_init(), parse the backend version list as comma-separated
tokens with kstrtouint(), keep strict token validation, and explicitly
require protocol version 1 to be present.

This replaces the deprecated simple_strtoul(), improves error reporting
consistency, and avoids partially parsed values in control paths.

Signed-off-by: Yufan Chen <ericterminal@gmail.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Message-ID: <20260324153023.86853-3-ericterminal@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>