xen/netback: use same error messages for same errors

Issue the same error message in case an illegal page boundary crossing
has been detected in both cases where this is tested.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Link: https://lore.kernel.org/r/20230329080259.14823-1-jgross@suse.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>